Add support for separate avconv parameters per output format

[displacedaussie]

This pull request resolves https://github.com/collective/wildcard.media/issues/16

It provides:

• Options for separate input/outfile parameters per output type
• The ability to force conversion of uploaded videos

With force set to True, this ensures that any uploaded videos will be converted with suitable conversion parameters (for example the baseline MP4 profile, or downscaled to a lower resolution) if required.

Without these options it's not possible to customise the output/conversion process for each file type or to guarantee that a video ends up in the desired format.

[vangheem]

Thanks for a PR. This is a lot of options now. Could you place all these options in a different fieldset?

[ivanteoh]

@vangheem We added fieldsets, rebased the branch and updated the changelog. Thanks again.