Allow JS as datafile

* `data: ‘data.js’` now works by `require`ing the JS file
* Test case, including one computed property
* Updated documentation
Minor clarifications for using JS module format

Changes some documentation and text strings surrounding the JS module
input format:

- differentiates use of dynamic module files in the introduction against
  using the static data formats
- clarify that URLs can only be used for JSON and YAML input, not for JS
  modules
- updates exception in _getDataFromFile() to mention JS modules as an
  acceptable input format
- adjust the unit test message for the JS module format
Stricter regex type checks in _getDataFromFile()

Instead of just matching anywhere in the string, provide an anchor so
that we are matching specifically filenames that *end* in `.json`,
`.yaml`, `.yml`, or `.js`.
Display warnings to console on bad/missing export

If the task encounters a JavaScript module which either exports a
non-object or does not export anything at all, display a warning to the
console.
Merge branch 'js-data'

Adds support to use a local JavaScript module as a data source using the
patch from @emilbayes.

Closes #26.
Merges and closes #27.
Bump mustache library dependency

Brings this up to v1.0.0.
Updated CHANGELOG, README, and package.json

Adds new entries to these for release of 1.8.0.