Specify page size when rendering

[istathar]

Add a --paper option to the technique render command to allow the user to specify the page size when rendering a Technique document to PDF. Accepts a4, a5, letter, or a width and height in millimetres.

[istathar]

This uses the implementation from

• Allow --size option to take dimensions harrington-free-press/cropped#3

but improves the --help output.