Parse error against negative sign

[gemmarx]

Parser cannot seem to handle negative sign.
Two evals below print "Can't parse: Minus" in stdout, and ErrorFunctionArgMismatch is returned.

engine.eval::<i64>("var n = -1; n");
engine.eval::<i64>("-1+5");

[gemmarx]

About this theme, issue #25 has been opened.
Thanks :-)

[luciusmagn]

I have already implemented unary operators in my local copy and hopefully, will be merging soon!