Allow macro definitions in settings (#493)

* 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

src/Settings.js

@@ -23,6 +23,7 @@ function Settings(options) {
     this.displayMode = get(options.displayMode, false);
     this.throwOnError = get(options.throwOnError, true);
     this.errorColor = get(options.errorColor, "#cc0000");
+    this.macros = options.macros || {};
 }
 
 module.exports = Settings;