Add dark theme

I've programmatically reversed the colors in `MarkdownEditor.tmTheme` and got a dark theme. Possibly fixes #76.

Here is the screenshot:

![screen 1](https://f.cloud.github.com/assets/844008/1383930/ce1b9188-3b45-11e3-914b-c9361e9ae6cd.png)

Some notes:

1. I didn't change the caret color. It's still blue (reverse was red).
2. I'm not sure if the selection color is good enough. Screenshot:
    ![screen 2](https://f.cloud.github.com/assets/844008/1383760/6cbfd686-3b42-11e3-8643-88e6dda5ea80.png)
3. I took care of RGBA colors (8 digit ones), too. No worry about them.
4. README should be updated to notify about the dark theme.