before_options for voice play_file/encode_file/encode_io

[abduct]

We have a way to provide options at the end of the ffmpeg command via the options variable, but no way to add options before the input.

Ffmpeg allows different options depending on the input file which can not be appended to the end of the command.

My suggestion is to implement a before_options variable which allows users to insert specific options before the -i input file specifier.