Cleanup @ynagorny's pull request

Hey guys,

I'm using headless in production for some scraping, and so I wanted to be able to use the built-in video screen capturing in production to provide evidence of bugs or site failure.

However on my development machine (arch), the default ffmpeg syntax doesn't work (because arch's version is developed by the ffmpeg team, not by the libuv team, which is the current default). In order to be able to test using headless I needed to be able to switch the command line arguments. Luckily @ynagorny's pull request works fine for me (see #46), but hasn't been accepted due to a few cleaning up tasks necessary for the merge. I decided to spend a quick 10 minutes to address these issues which I hope will lead to this change being merged & deployed, so I don't need to refer to my own fork in order to continue using this gem.

In #46, @abotalov mentions a miscoception I'd like to address to alleviate any worries about incompatibility:

 > This is a public API breaking change so if it will be merged with this change next headless version should be major according to semver. Previously ffmpeg was used. If you really want to make this change you should change occurences of ffmpeg in Capturing video section too.

The ffmpeg-built version of ffmpeg is not the default in the currently released gem - it's the libav-built version of ffmpeg. This pull request doesn't change that, it only adds the option for users to switch over to the ffmpeg-built version if they choose - for this reason, this pull request should not break compatibility.


If you've any questions, please let me know. Let's get this merged! :smile: