Add ability to recurse through subdirectories

[agude]

Add a --recurse (or -R) option that recurses through subdirectories and finds video files in them. It assumes that all the files in the directory and subdirectories are from the same series. The files in subdirectories are renamed in place.

This resolves #8.

[agude]

I tested this on a mocked up directory as follows:

├── s3e1.mkv
├── season1
│   ├── s1e1.mkv
│   └── s1e2.mkv
└── season2
    ├── s2e1.mkv
    └── s2e2.mkv

After running with '-R' it looks like:

.
├── season1
│   ├── The West Wing [1x1] - Pilot.mkv
│   └── The West Wing [1x2] - Post Hoc, Ergo Propter Hoc.mkv
├── season2
│   ├── The West Wing [2x1] - In the Shadow of Two Gunmen (1).mkv
│   └── The West Wing [2x2] - In the Shadow of Two Gunmen (2).mkv
└── The West Wing [3x1] - Manchester (1).mkv

Please give it a test for your own cases before merging though!