Implement tab width and the draw_white_space setting

[erbridge]

See limetext/lime#300.

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/33648128-implement-tab-width-and-the-draw_white_space-setting?utm_campaign=plugin&utm_content=tracker%2F15716642&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F15716642&utm_medium=issues&utm_source=github).