feat(index.js): Add range notation support

[fabiancook]

Resolves issue #1

[coveralls]

Coverage remained the same at 100.0% when pulling e71543d on fabiancook:feature/cell-ranges into 0b8d147 on mike182uk:master.

[fabiancook]

It should be noted that detectRange has been included so that no existing code is broken, someone may be catching the error created and performing a different action.

toA1Range and toR1C1Range have been included for people who want to specifically convert ranges.

If no type is passed to range then it will auto-detect, however the two cell references should be the same, this is only an artificial limitation and if you think we should allow different types then we can change that.

[coveralls]

Coverage remained the same at 100.0% when pulling ee39e31 on fabiancook:feature/cell-ranges into 0b8d147 on mike182uk:master.

[mike182uk]

Hey @fabiancook ,

Thanks for contributing this 😃

I've taken a look at your implementation and it looks to do the job which is great 👍

I think we can do a little refactoring though. I'am not keen on the the extra boolean param thats been added to most of the methods - i can see why you have done this, but i think it makes things a little more complicated. I am happy to bump the version to account for BC breaks if we need to.

On a sidenote this, is a good read on boolean params.

I really like your idea of passing a convertor to the convertRange method, nice way of reusing existing functionality.

Based on your changes, i think we could simplify things to something like the following:

module.exports.toA1Range = // ...
module.exports.toR1C1Range = // ...

var R1C1RANGE = // ...
var A1RANGE = // ...

function cellref(ref) {
  if (R1C1RANGE.test(ref)) {
    return convertRange(ref, convertR1C1toA1);
  }

  if (A1RANGE.test(ref)) {
    return convertRange(ref, convertA1toR1C1);
  }

  // ...
}

function convertRange(range, converter) {
    return range.split(':')
        .map(converter)
        .join(':');
}

function convertR1C1RangeToA1Range(range) {
    // check range is valid...

    return convertRange(range, convertR1C1ToA1);
}

function convertA1RangeToR1C1Range(range) {
    // check range is valid...

    return convertRange(range, convertR1C1ToA1);
}

This would then leave cellref to do the autodetecting and get rid of the boolean flags.

What do you think?