Although we'll now end up with only `mustache.js` in the git repo,
being written in ESM syntax, the npm package will still be as before:

- `mustache.js`: UMD, meaning CommonJS, AMD or global scope
- `mustache.mjs`: ESM

Covers the project with browser tests, not only V8 via node/iojs. Tests are runned with [zuul](https://github.com/defunctzombie/zuul) which
provides simple local browser testing, aswell as cloud testing on multiple browsers with [saucelabs](http://saucelabs.com).

+ minor fix for better error message when running specific render test

Fixes #415

This commit is a complete rewrite of the core mustache.js file with two
main goals: 1) a major performance boost and 2) better compliance with
the mustache spec.

In order to improve performance templates are pre-compiled to JavaScript
functions. These compiled functions take a view, a partials object, and
an optional callback as arguments. They are cached to prevent
unnecessary re-compilation of an already compiled template. Both of
these enhancements facilitate a generous boost in performance.

A few other notes:

- The mustache.js file is now both browser and CommonJS ready without
  any modification.
- The API exposes two main methods: Mustache.compile and
  Mustache.render. The former is used to generate a function for a given
  template, while the latter is a higher-level function that is used to
  compile and render a template in one shot. Mustache.to_html is still
  available for backwards compatibility.
- The concept of pragmas is removed to conform more closely to the
  original mustache spec. The dot symbol still works to reference the
  current item in an array.
- The parser is much more strict about whitespace than it was before. The
  rule is simple: if a line contains only a non-variable tag (i.e. not
  {{tag}} or {{{tag}}}) and whitespace, that line is ignored in the
  output. Users may use the "space" option when compiling templates to
  preserve every whitespace character in the original template.
- The parser is able to provide detailed information about where errors
  occur when parsing and rendering templates, including the line number
  and surrounding code context.

lib/mustache.js is now always kept in sync with mustache.js

(cc @tlrobinson, @boucher)