Add custom environment variable via ScriptOptions

+ Fix #145
Definitely. Using `VALID_VARIABLE_NAME` vs. catching a "wildcard" (exc. quotes) token is a temporary work around for the issue I was seeing evaluating the grammar. When I added a proper wildcard token it would collide with the `VALID_VARIABLE_NAME` and partial matches would occur on that. If I made the wildcard token higher precedence it would begin to each captures of `VALID_VARIABLE_NAME`. I'll probably get to trying to update the grammar this weekend myself. I seen similar issues with escaped quotes, though I remember having a bit more success with them.