Small api cleanup

This pull request includes several small changes to clean up the API and add minor features. Tell me if you'd prefer me to submit a separate pull request for each of those.

* `withVideoBitrate` now accepts a boolean argument for the constant bitrate flag. The old CONSTANT_BITRATE and VARIABLE_BITRATE constants are still supported.
* `usingPreset` accepts functions (this does not conflict with #208)
* added `addInputOption` and `addInputOptions` to mirror `addOption` and  `addOptions` for input streams/files
* keep fractional seconds part when parsing ffmpeg timestamps (fixes #81)
* added `withAudioFilter` and `withVideoFilter` to specify custom filters

All those are documented and tested.