Add base16-spacemacs theme

[22a]

Adds a base16-spacemacs theme. It follows the same primary mode colours as molokai {normal:blue, insert:green, visual:yellow/orange, replace:red} but with the colour palette of the spacemacs variant of the base16 themeset.

Mode run-through:

Normal mode with trailing whitespace:

Normal mode with unwritten file modification:

Insert mode:

Visual mode:

Replace mode:

Inactive mode:

[chrisbra]

thanks.