Mini-notation additions towards tidal compatibility (#926)

- `.` feet now work, e.g. `"a b c . d e . f"` is the same as `"[a b c] [d e] [f]"`
- `_` elongation now works, e.g. `"a _ _ b"` is the same as `"a@3 b"` (`"a @ @ b"` also works, like in tidal)
- standalone `!` now works, e.g. `"a ! ! b"` is the same as `"a!3 b"`

Reworks #47 and #49
Ref #30