Highlight misspelled words only if they are likely to be prose

The default mode highlights only words in strings in comments.  An
alternate markup language mode highlight most text, but leaves out
formatting commands.  This is handled in CSS; spell checking is still
performed on each word.

As a result, we can remove the hack to exclude TeX macros.

Refs #5.
Avoid multiple calls of getSuggestions
Only provide spelling suggestions for highlighted words

We essentially repeat the CSS logic for whether to show the dotted line.
This means that the lists of tokens is duplicated, so we have to
remember to update both.  Perhaps there's a better way to handle this.

Closes #5.
Version 0.0.4