Add support for multiple source directories to the hologram config

- Source now allows either a single value (keeping backwards compatibility)
  or a yaml array of sources.
- This is useful when pulling in a library that comes with Hologram docs
  support built in.
- We considered auto resolving conflicting namespacing for multiple
  libraries, but we weren't sure that was really an essential feature.
  We'll use this feature for a bit, and add it if it seems worth the
  effort.
- We didn't modify the CLI to take multiple sources, if you'd like that
  feature, we're happy to add it.
Merge pull request #131 from stubbornella/multi-source

Add support for multiple source directories to the hologram config
Add cortana theme to readme extensions and plugins
Relax version requirement for redcarpet
Remove deprecation warnings in test suite
Revert "Remove deprecation warnings in test suite"

This reverts commit adffdf55dd8fced5e7774bc3aa1d32983ea0c43d.
Fix nil on potentially empty dependency config var
Removing deprecation warning about be_true and be_false
Merge pull request #138 from carpodaster/fix/relax-redcarpet-dependencies

Relax redcarpet dependencies
Add simple plugin support
Update tests for plugin support
Pass extra flags to plugins, use plugin instances instead of static calls
Add plugin class

Add Hologram::Plugin that other plugins can extend from. This takes
takes care of the following cases:

- Checks arguments to see if the plugin is active (is_active? = true)
- Replaces plugin text when disabled
- Plugins get their text from [[[plugin:plugin-name ... ]]] blocks. This
is done automatically by the base plugin class. The .plugin method can
return either an emtpy string or some other text that will be put in the
plugin block's place.
Merge pull request #142 from ramyathulasingam/master

Removing deprecation warning about be_true and be_false
Replace pygments with rouge
Merge pull request #121 from jdcantrell/plugins

Plugins
Merge pull request #141 from jdcantrell/rouge

Replace pygments with rouge
Update version to 1.2