Added support for using v4l2 as input format

FFmpeg outputs two v4l2 formats
```
"v4l2":{"description":"Video4Linux2 output device","canDemux":false,"canMux":true},
"video4linux2,v4l2":{"description":"Video4Linux2 device grab","canDemux":true,"canMux":false}
```
and this results in fluent-ffmpeg only accepting ```-f video4linux2,v4l2``` as the v4l2 input format. I've set the first output's canDemux to true in order to recognize ```-f v4l2``` as a valid input format. 
You could also change the regex to recognize formats with commas in them but this is a quick fix.