Prevent infinite loop from empty match (fix #77)

[jonathanhefner]

This fix for #77 prevents empty regex matches (via e.g. /a?/, /a*/) from causing an infinite loop. It does so by checking that the regex's lastIndex property increases on each call to RegExp.exec.

Incidentally, RegExp.exec sucks and mutation is evil, so I included another fix to prevent said mutation from leaking into Elm.

Test coverage is included.