Implement custom foreground/background color for current line

Added a configuration option for the current cursor line's foreground and background colors:

![fonts_and_colors](https://cloud.githubusercontent.com/assets/90451/5572557/6114c176-8f6e-11e4-8ec8-52c422774b39.png)

Render the current cursor line with the custom settings:

![current_line](https://cloud.githubusercontent.com/assets/90451/5572535/dd68e1e0-8f6d-11e4-93f2-f4f944009952.png)

The current cursor line will fall back to the "Line Numbers" colors if values are not specified.

Thanks for the fantastic extension!