Add support for custom gst pipelines (#40)

* add custom pipeline parsing

* add custom pipeline support to CLI interface

* add appsrces to pipeline, add example

* add support for custom pipelines, add EOS signal support, add first example of mp4 pipeline (audio does not work here, will figure out why)

* add constants to linux code

* move fixes to linux code

* move fixes to linux code

* add working example for writing to mp4 file

* add working mac os example

* add hack to wait long enough for MOOV box to be written, add working linux example

* wait for EOS signal instead of sleeping a second

* add Stop to interface, increase timeout

* increase timeout

* fix test

* bump version, update readme