helptext: add support for custom banner message

This adds the methods `setBanner()` and `getBanner()` to allow customization of the banner message itself.

Example scenario: I write an application which accepts an arbitrary number of files ("operands" in your terms) but it's hard to properly convey to this user. The would allow me to change the message e.g. to:
`Usage: myscript [options] [operands]` to
`Usage: myscript [options] <files...>`
etc.