"Check" Mode

[t-rad679]

Imaginemin currently only performs fixes...this is unusable for automated presubmit (pre-merge) checks. Although I am admittedly not very good with JS, I get the impression it would be pretty straightforward to add a parameter-controlled mode that compares the result of compression to the original file instead of writing to it. If the two are different, it would simply return in error.

If I find time, I will make the change and create a pull request. Allow me to reiterate, though, that I am inexperienced with JS and my implementation is likely to be ugly.