Add jzintv --help to show features / switches

[intvsteve]

From user mthompson (and also informally mulled over for quite some time), this would be a really helpful feature - especially when configuring jzintv!

Details:
Consider a modeless window that could be accessed directly in LUI and kept around (just moved to the side)
Concern: behavior when dialogs appear
Windows: Floating window?
Mac: Which layer?
GTK: Requires investigation

General: Present stdout using a monospace font, scroll if necessary, support copy, not editable.