Support basic mathematical operations on the grammer

[vivekmittal]

It would be great if basic mathematical operations can be accommodated in the grammar.

E.g.

x+4 eq 10
x%20 eq 1

[nikunjy]

I am happy to extend if you wanna submit a PR.

[nikunjy]

Just remembered this issue. You could lift the grammar from here https://github.com/nikunjy/antlr-calc-go