Provide mechanism for general description text, that is not specific to any option or argument

[claremacrae]

For background, see twitter discussion here: https://twitter.com/ClareMacraeUK/status/949415455399268353

Summary: It would be great to be able provide some descriptive prose to the parser, and have that nicely formatted and included when writing out help text.

My case is that I'm writing out Clara's usage text twice - once on error, and once on --help, and I'd like to insert some text between Clara's "usage:" and "where the options are:" lines.

[parnmatt]

:+1: