Allow files as inputs

[rec]

So people could run recs on existing recordings.

[rec]

This could be a huge and expensive feature if it interoperated with the rest of the program, but I think 90% of the value could be had for 10% of the work by allowing a completely separate mode where recs ran as fast as possible on just a single mono or stereo file in process, in a single thread and process and no other inputs were active at the same time, but all the other flags/keywords were active.

[rec]

This might be worth doing because of how it makes #116 possible to mostly do automatically as a unit test, though it's not a full end-to-end test without going through an audio interface...

[rec]

It would be easier to do the whole thing!

New protocol or base class Source.

InputDevice and new FileReader would be instances of it.

Tracks would take a Source.

The only issues are two:

1. Setting the name of the output file
2. If there is no type or format set, we need to read the type or format from the original file and use that.

[rec]

• [x] Abstract the Source protocol
• [x] Allow Update to have a None timestamp if this is easy
• [x] Turn Path into an Option again
• [x] Add --files or --input or something like that Option
• [x] If files are selected, this turns off the audio receiving entirely, which also turns off the device_name job

[rec]

Almost done! A new list:

• [ ] Write and test code for the output filename if input files are selected
• [ ] Write and create two test .wav files, one mono, one stereo
• [ ] Add an end-to-end test