This changes the base16 theme back from #RRGGBB0f to #RRGGBB00,
reverting part of #934. That PR used the 0f encoding to produce ANSI
escape sequences 30-37 and 40-47 rather than 38;5 and 48;5 which require
256-color support. Unfortunately, it resulted in base16 using the wrong
colors becuase ansi_term does not support the bright variants (90-97 and
100-107) so it simply mapped them to the non-bright colors.
This PR makes combines the 00 and 0f alpha encodings into 00, and makes
them use the Color enum for the first 8 colors and Fixed otherwise. This
means the ansi-light and ansi-dark themes will work on terminals without
256-color support, and base16 will render bright colors correctly.
