Add i3 bindings and segments
Use 'with' instead of lock.acquire/release()
Improved battery segment:

- Ignore `steps` if gamify is False.
- Add `full_heart` and `empty_heart` keywords.
- Document `format` keyword.
- Replace `draw_soft_divider` with `draw_inner_divider`.
Fix docstring for i3wm.workspaces segment
Remove dead code from renderers/i3bgbar.py
Adapt to python3 / __future__ print statement
Fix highlighting groups for workspaces segment
Update documentation to include i3 instructions