initial implementation of \textcolor
add a test for nested \textcolor
rm a console.log :\
reformat textcolor test case to rm <hr/>
comment \textcolor with links to docs
allow whitespace between \textcolor and 1st block

note that whitespace within the first block will be ignored by the CSS
color property, so we don't need to worry about that.
Merge pull request #151 from mathquill/feature.textcolor

Implement \textcolor