Allow spaces and comments after continuation

Continuations are used to break up complex logic and I find it frustrating being unable to use comments in them. This change to the grammar serves two purposes:

1. it removes the shell scripting legacy of not being able to have trailing spaces
2. allows comments between the continuation and the next expression

```boo
    if Path.Exists(fpath) and \
       check_header(fpath) and \   # comment
       # not_working(fpath) and \
       is_valid():
        return True