Refactor to make adding more CSS props simple; add support for variable character width & letter-spacing

[tinymachine]

Had a bit too much spare time, so I may have gone overboard with this PR. Totally understand if you need to reject it if it's too broad.

Major change: apply defaults, calculate and adjust property values programmatically

The biggest update here is in 7d26406, which refactors how CSS properties are handled by the script. Instead of 'manually' handling all calculations (with separate lines of code for applying defaults for each property, and calculating and applying each property value), all supported properties (and any defaults) are defined in a config var. Then defaults are applied automatically for all included properties, and all calculations are handled programmatically. This way when you want to support additional CSS properties, you just update the config, without having to worry about making additional per-property updates throughout the script.

For example, adding letter-spacing support to the script is now as simple as adding the following to config.supportedProps:

{
  propName: 'letterSpacing',
  unitsDefault: 'em'
}

Adding per-property defaults for minWidth and maxWidth is supported as well, of course. Here's the entire config for all five props supported in the PR:

supportedProps: [
  {
    propName: 'fontSize',
    minWidthDefault: 1.0,
    maxWidthDefault: 1.8,
    unitsDefault: 'em'
  },
  {
    propName: 'fontStretch',
    unitsDefault: '%'
  },
  {
    propName: 'fontWeight'
  },
  {
    propName: 'letterSpacing',
    unitsDefault: 'em'
  },
  {
    propName: 'lineHeight',
    minWidthDefault: 1.33,
    maxWidthDefault: 1.25
  }
]

Limitations

1. Each supported property must have a propName that matches the JS equivalents for CSS properties. The script assumes the propName is what's used by the API params, so the letter-spacing params would (as currently coded) have to be min/maxWidthLetterSpacing (the script is aware of the initial cap on the property name) and letterSpacingUnits. Params for changing character width would (as currently coded) have to be min/maxWidthFontStretch and fontStretchUnits. This could be changed pretty easily to handle param names that don't necessarily map directly to their CSS property, but it might be a good thing to enforce a standard style on the params and to make explicit exactly what the params control.

2. Special properties like font-variation-settings, which can control multiple properties at once, aren't supported. This is probably fine, though, since the VF variation axes users will probably generally want to control (and the ones more variable fonts support) would be more standardized variation axes like weight and sometimes width, which honor their own respective CSS properties (e.g. font-weight and font-stretch).

Other updates

• Deprecated min/maxWidthVariableGrade in favor of min/maxWidthFontWeight (closes Though params refer to variable font grade, script changes font weight #29) (also related to limitation 1 above)
• Replaced use of font-variation-settings for weight with font-weight so the cascade is better respected (see commit message for 736e4af) (also related to limitation 2 above)
• Replaced units with fontSizeUnits (but units still honored), add support for per-property units (closes Rename units param to fontSizeUnits? #23)
• Replaced date with version number in textblock.min.js (closes Echo version number to textblock.min.js? #31)
• Added support for variable font-width and letter-spacing (closes Add variable font character width adjustment #22 and Add letter spacing adjustment #28)
• Improved grunt compression (though this update contains a little more code than previous versions, the minified script weighs in at 1.71 KB, down from 1.84 KB)

[tinymachine]

Sorry, squashing some commits to make this PR easier to review. Will re-submit it later.