Support fetching title (and other video-level metadata)

I'm working with some video data where some of the actual label information is encoded in metadata within the (mkv/mp4) container format).

From #24 I see we're now able to access the *rotation* of a video at Python level, but it seems like this is a one-off implementation that doesn't expose any other metadata fields?

At the moment I'm specifically looking for `title` and `comment` tags, but as far as I understand the container specs are pretty open-ended - so maybe a general accessor for string metadata would be useful?