Add recurse option

[oir]

A first attempt at recursive parsing. Forces children Args to be keyword-only, and not have any n-ary or variadic parameters. Option naming is flat, not hierarchical.

Related: #67, #66

• handle help
• handle name conflicts between parent and children
• handle short names
• more negative tests covering short name collisions

[coveralls]

coverage: 98.936% (+0.4%) from 98.541%
when pulling 1a02ea0 on recurse
into 2abd464 on main.