Dmslogo

Added
- Ability to shade stacks in `draw_logo`.

Added
- Additional ways set y-axis limits. This involves adding `utils.AxLimSetter`, and adding the `ylim_setter` param to `draw_logo` / `draw_line`. It also involves enabling `set_ylims` in `facet_plot` to take per-row values. Finally, a new example on y-limit setting was added.
- Allow `show_color` for `draw_line` to be `None`.

Fixed
- Updated how fontlist is built to fix [this matplotlib warning](https://github.com/matplotlib/matplotlib/issues/17568).
- Fixed deprecation warnings.

Fixed
- Fix bug in error checks when `x_col` isn't sequential sites.

Changed
- Put the mid-line on logo plots from `draw_logo` with negative values on top of text.

Added
- Negative values can be plotted using both `draw_logo` and `draw_line` (as well as `facet_plot`). A new example Jupyter notebook was added for such plotting.

Added
- Example notebooks now run on `mybinder`.

Changed
- Examples now in Jupyter notebooks and converted to docs via `nbpshinx`.
- Tests now run on Python 3.7 as well as 3.6.
- Some improvements to docs.

Added
- Option to fix y-limits in `facet_plot`.