Pypianoroll

- Add `pypianoroll.metric` module
- Support more options in `pypianoroll.parse`
- Rename methods for `Multitrack`
- `get_maximal_length` -> `get_max_length`
- `get_num_downbeat` -> `count_downbeat`
- `get_stacked_pianorolls` -> `get_stacked_pianoroll`
- Improve the documentation (mostly docstrings)

- Make Matplotlib and Moviepy optional
- Add unit tests
- Fix note off bugs in `pypianoroll.Mutitrack.parse_pretty_midi()`
- Move all plotting functions to one file (`pypianoroll/plot.py`)
- Remove normalization arguments from plotting functions (value range now infered from dtype)
- Add argument `dtype` to `pypianoroll.Track.assign_constant()`

- Fix downbeat loading bug
- Fix wrong usage of np.issubdtype()
- Fix wrong error types

- Change default mode for MultiTrack.parse_midi() and MultiTrack.parse_pretty_midi() to 'max'
- Remove mode 'any' from MultiTrack.parse_midi() and MultiTrack.parse_pretty_midi()
- Use np.uint8 for the piano-rolls parsed from MIDI files when possible to save disk space

- Fix 1 and 2
- Change argument names and orders in plot functions
- Use intersphinx for documentation