[TODO] Slightly better hyphen support

[mesalu]

Current grammar doesn't support a hyphen as part of a term (which becomes relevant on the very first term, or on terms following an operator), so things like the following would be rejected:

• -2 + d20
• 2d10 + -5

The grammar should be reworked (and how the bot acts on the parse nodes for that matter) to permit tighter binding of the hyphen.
(Negative n-values for diecasts should still be illegal.)