add red underline to mistyped whitespace

[fjl]

This makes errors where a character was typed instead of space visible.

[hiaw]

+1 to accept this pull request please!

[jasonm23]

I added a similar solution in #17 ( added after yours @fjl )

paging @hagleitn

[jasonm23]

@fjl since Gunther doesn't seem to be actively interested in speed type at the moment, I'm thinking of making my patch an add-on package e.g. speed-type-error-highlighter

It seems to me that your strategy of marking the char as correct (or not) and abstracting away the color / face change makes more sense. (I just add on a bg-color at the moment)

I think it'd be better if we simply made a patch which added speed-type-correct-face and speed-type-incorrect-face and applied them based on the value of correct. This way themes could override the defaults (etc.)

Any thoughts?