R158 – road to α, step 2

Fixed the minus problem so to be able to draw southern hemisphere constellations:

• R158 (Apr 26, 2013): Implement negative constants

The problem was fixed by creating a rule:

⟨constexpr⟩ ::= ('+'|'-') ⟨numexpr⟩

by the function parse_constexpr. I was forced into accepting the idea of using token ** args to by side effects appoint constants.