allow other options to ffmpeg
add section to README for new options
allow custom size filters
document filter option
typo
additional flags to globally apply to all encodes
docs for global flags
Merge branch 'customoptions'

* customoptions:
  docs for global flags
  additional flags to globally apply to all encodes
  typo
  document filter option
  allow custom size filters
  add section to README for new options
  allow other options to ffmpeg
correct test media
basic test for filter option