match-highlight: Formatting, optimizations and fixes
Fix "the NULL class" bug

(yeah, I've just invented the name ;)
Since a plaintext's style is always `null`,
when it gets concatenated with `spanStyle`
it converts to the string "null", which is later
applied as classname.

Don't believe me? Go open
http://codemirror.net/demo/matchhighlighter.html
then select `hardSpotVar` and inspect the lines' DOM.