Scope usage lines

[victor-gp]

In the work for #23, it became apparent that not handling usage lines blocks the handling other patterns.

In particular, most patterns that start with a subcommand (lowercase name), such as those reported in #3

It should be easy enough, here are some heuristics off the top of my head:

• after section headings like "usage:" or "examples:"
• first few lines at the beginning of the help message
• stop handling them as usage lines after the first double break (two consecutive \n's)

[victor-gp]

Some examples: