Expose ffmpeg child-process & allow URL paths

[tan-tan-kanarek]

The purpose of these changes is to expose ffmpeg process to higher level in order to log the ffmpeg command using application logging framework and in order to have better control on the child process for piping std in, out and err or to kill the process when needed.

Also support remote input files over http

[tan-tan-kanarek]

merged 0.5.0

[maksym-plotnikov]

@tan-tan-kanarek could be also useful if You add onProgress callback function to pass overall progress output to higher level.

[tan-tan-kanarek]

It's a good idea, but not something I need at the moment because I use the canvas renderer where I'm already well aware to the progress.
Are there any comments or improvements I can do to promote this PR to be merged?

[mifi]

This PR is two features in one (allow http/https + onStart / onProcessStart)