* Introduce MacroExpander
The job of the MacroExpander is turning a stream of possibly expandable
tokens, as obtained from the Lexer, into a stream of non-expandable tokens
(in KaTeX, even though they may well be expandable in TeX) which can be
processed by the Parser. The challenge here is that we don't have
mode-specific lexer implementations any more, so we need to do everything on
the token level, including reassembly of sizes and colors.
* Make macros available in development server
Now one can specify macro definitions like \foo=bar as part of the query
string and use these macros in the formula being typeset.
* Add tests for macro expansions
* Handle end of input in special groups
This avoids an infinite loop if input ends prematurely.
* Simplify parseSpecialGroup
The parseSpecialGroup methos now returns a single token spanning the whole
special group, and leaves matching that string against a suitable regular
expression to whoever is calling the method. Suggested by @cbreeden.
* Incorporate review suggestions
Add improvements suggested by Kevin Barabash during review.
* Input range sanity checks
Ensure that both tokens of a token range come from the same lexer,
and that the range has a non-negative length.
* Improved wording of two comments
