This changes the output color of the grid and the line numbers to use
the "gutter" foreground color defined in the Sublime `.tmTheme` files.

Sublime Text does the same.

Note: we could go one step further and also extract the "GitGutter"
colors from the themes. These could be used instead of red/green/yellow
to signify Git modifications. The problem is that they are quite a bit
harder to extract from the syntect `Theme` object.

closes #178