Improve keep_audio and keep_video API

[sleepycatcoding]

Makes keep_video and keep_audio API more useful by allowing consumer to specifiy paths where files should be saved.

TODO

• Update documentation

[emarsden]

Thanks, the general principle looks good to me.

A nitpick, perhaps the "Other" error could be made an IoError with a reason similar to "requested audio path is invalid UTF-8"?

[sleepycatcoding]

The problem of making it a IoError is that we need a std::io::Error object.

https://github.com/emarsden/dash-mpd-rs/blob/6a73f89438ab43fa30a63091f7f299b48b0e11f9/src/lib.rs#L106-L107

[emarsden]

OK, thanks!