Add theme support

[peter-liu]

It will co-work with instant-markdown/instant-markdown-d#90

it looks like (theme=night):

[ashwinvis]

I would prefer the dark instead of night theme. I think the phrase "dark theme" is more common. What do you think?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[ashwinvis]

It is almost ready. I think I will nedd to make the fixes myself.

[ashwinvis]

The night has been renamed to dark theme.