FFmpeg custom arguments

[amishshah]

If you'd like to pass custom FFmpeg arguments, you need to create your own FFmpeg transcoder stream. This can be annoying, especially for those who would like to use reconnect arguments (see #117 and #141)

There should be a way for users to override the FFmpeg arguments provided by the library. This could also be a more general issue of allowing users to customise the transformer graph themselves.

This isn't a simple fix and will require a bit of thinking to come up with a nice solution.

[iim-ayush]

#124 I asked earlier also.