Add filter capability helper

In this pull request:
* Added a new getAvailableFilters method
* Enabled calling getAvailable* methods directly on the module without instanciating a command (eg. `require("fluent-ffmpeg").getAvailableCodecs(callback);`)
* Added tests for getAvailable* methods
* Fixed a bug where "=" was wrongly reported as an available codec
* Added documentation for getAvailable*
* Added myself to contributors :)