Added support for truecolor Neovim and for the Neovim terminal

[sp5]

I replaced several calls to has('gui_running') with a function to check if we're running in Neovim and, if so, check if it can use truecolor.

The colors for Neovim's :terminal were matched up to the ones for gnome-terminal

[rawaludin]

This should be merged.

[SuperBo]

This absolutely should be merged!