Rely on the 256 Colour Mode for Consistency

Because vim is used on both Linux and OSX (iTerm2), just rely on using
vim's 256 colors rather than trying to filter through the terminal
colours.

There might be a way to support the terminal colours depending on which
environment this is running on.
Switch Hybrid Color Terminal Settings

Based on whether Vim is running on Linux or OSX (iTerm2), set the hybrid
global variable to allow the terminal colours to come through.

On both Linux and iTerm2, the terminal colors are set from:

    https://github.com/w0ng/vim-hybrid