Lookahead support

[artamonovkirill]

We've been using exrex for NL postal codes generation using the following regex: ^(?i)[1-9][0-9]{3} ?(?!sa|sd|ss)[a-z]{2}$.

Looks like lookahead assertions are not being applied correctly: https://github.com/artamonovkirill/exrex/blob/test/lookahead/tests.py#L67 (after I doubled the amount of tests, it fails almost 100%).