Transcoder: add process method based on stream

* Enum to value eProcessMethodBasedOnStream.
* Add attribute _indexBasedStream to get the index of stream.
* Stop transcode at the end of an indicated stream.
Transcoder: rename _indexBasedStream to _mainStreamIndex

Index of stream used to stop the process of transcode in case of
eProcessMethodBasedOnStream.
Merge pull request #106 from cchampet/feature/FTDMAT-326

Transcoder: add process method based on stream