Better transparent terminal support

Sets the background color in cterm and gui to 0 and #000000 when
g:zenburn_transparent is 1.

Signed-off-by: Jani Nurminen <slinky@iki.fi>
Sync with Kim Plowright's contrib repository

Contains:
  02b59e470e08bd642af05239afba368206fd1c3a adding alfred app theme
  699e1d7246c9fe1139f4b0f718c184ca0ae4631b Adding ports for mou and omnifocus

Signed-off-by: Jani Nurminen <slinky@iki.fi>