[FR, video-ffmpeg] Allow custom framerate

[Egocentrix]

Right now the ffmpeg script allows you to select the framerate from a combobox, but this list is limited. I think it would be nice to be able to set a custom framerate, for instance for slideshows or low framerate GIFs.

[supertobi]

I've added more options, because I was to lazy to change the filed type. Pull request for an input field are welcome, but then please add some kind of check.

This is the new list:
framerates = {"1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "15", "16", "23.98", "24", "25", "29,97", "30", "48", "50", "59.94", "60", "120", "240", "300"}

[Egocentrix]

Haha, that works. Thanks! I don't really speak Lua, but might try my hand at this some time.