implement TextColor::latex
generate HTML template, don't mutate DOM in jQadd

a la #192

Note that the whitelist of allowed characters (to avoid injections)
includes dash, which shouldn't appear in normal CSS color values, but I
include it because negative numbers are specifically given as an example
of a useable color value, see Example IV of
http://www.w3.org/TR/2011/REC-css3-color-20110607/#rgb-color