Fixtures for partials in subdirectories

Adds to test/fixtures/partials/sub-a and test/fixtures/partials/sub-b
for future unit tests involving testing partial references that use
directories rather than just bare filenames.

Included in each subdirectory are three partials analogous to those
found in the parent directory.
Existing directory-with-prefix behavior unit test

Introduces a new test to the testing suite that combines the existing
prefix option with a partial reference that uses a subdirectory. The aim
of this test of to ensure that the (arguably weird) behavior of applying
a prefix to the directory works in future versions of the plug-in after
changes are made to the prefix handling.
Partial reference using long path unit test

Introduces another unit test for the purposes of making sure that a user
who specifies a partial relative to the base of their project (i.e. such
a reference where they have left the 'directory' option blank) gets the
correct result with the upcoming prefix changes.
directoryPrefix and filenamePrefix support

Adds support for these new options, including mapping usage of the
old-style prefix option to the new option depending on what kind of
partial reference is encountered.

For bare filenames, prefix behaves like filenamePrefix. For paths
including a directory, prefix behaves like directoryPrefix (which
matches the current weird behavior).
Unit tests for new xxxPrefix options

Adds unit tests for filenamePrefix, directoryPrefix, and a combination
of the two. Reuses existing fixture data and expectation output, except
for the combination unit test which uses new expectation output.
Warnings for weird partial references

If the user attempts to use a partial reference that does not exist or
tries to use the old-style prefix option with a path, prints a warning
to the terminal.
Warning on bare filename w/ directoryPrefix

Prints a warning to the terminal if the plug-in encounters a bare
filename partial in a ruleset that has specified a directoryPrefix.
Update README for new options

Adds directoryPrefix and filenamePrefix to the options list, replacing
the old prefix option.
Rename new options to use underscores

Switches directoryPrefix to prefix_dir and filenamePrefix to
prefix_file, which more closely matches the existing naming convention
used by the other options. By renaming directoryPrefix, this also avoids
the inherent confusion with the unrelated 'directory' option.
Remove unused partial fixtures

I did not end up using sub-b/ and also do not test alternate extensions
in combination, these files can safely be removed.
Merge branch 'prefix-expansion'

Introduces prefix_file and prefix_dir as separate options for looking up
the paths of partials. The original prefix option is maintained for
backward-compatibility.

Fixes #9.
Closes #21.
Dependency update, except mustache

Brings all dependencies, both for development and running, to their
latest versions, except for the mustache library.

All unit tests pass.

The mustache library has a breaking change in the 0.8.x version, as it
saw the removal of the compile() function. This will be handled in a
separate commit.
Mustache library dependency update

By updating to Mustache 0.8.x, the compile() function goes away. In this
version, render() knows how to cache templates, so no longer need (or
want) to precompile the templates to yield render functions.

As such, the "renderFn" and "compile" functions have been renamed,
notably:

- getRenderFn becomes getBody
- compileTemplate becomes getBodyFromFile
- getRenderFnFromUrl becomes getBodyFromUrl