Attempt to resolve a valid regex

- Only do so if the current token is DIV and the previous token falls
  under one of the affected ambiguous token types.
- Also corrected the tracking of previous token to be more useful for
  error reporting, as part of the fix to what that lexer variable
  tracks.
- Based on hints at: https://stackoverflow.com/a/27120110/2904896

Fixes #25.