Tweaks Ruby, diff, and parentheses highlighting.

[kern]

I tweaked a couple of the odd parts of the Tomorrow Night color scheme. Here are a list of the modifications.

• Gave matched parentheses the selection background instead of the ugly default color.
• Sets TODO's background to the background color.
• Highlight the string delimiters in Ruby green like the string.
• Highlight string interpolation characters orange.
• Highlight diff additions green.
• Highlight diff removals red.