Add support for custom position labels

[julianandrews]

It would be nice to have a way to specify the labels to use.

Implementation should be easy, but the CLI UI design is a little tricky. I think I probably want independent flags for the --x-labels and --y-labels but I'm not sure what they should look like.

Ideally, I'd like to support

• A fixed list of labels or
• Ranges of characters (something like A-Z)
• Numbers (same as ranges of characters?)

Besides figuring out what an argument that allows that would look like, I also have to worry about label length. Options:

• Only allow single unicode character labels
• Resize margins to allow longer labels, and display x-label text vertically

That second option is definitely a lot more work, and finicky (since SVG 1.1 text rendering isn't the most graceful), so at least for the first version of this, I'm thinking to limit labels to one character.

This should be easy enough to implement once I decide how to answer these questions, but I'm not really happy with any of the UI options I can think of.

This is split off from #12.