Colour customization

[akabiru]

Users should be able to set custom colours on the prompt. E.g. panda, git:(), ft-test-#127978433.

Example variable names.

$RAFIKI_FOLDER_COLOR='cyan'
$RAFIKI_GIT_WRAPPER_COLOR='blue'
$RAFIKI_GIT_BRANCH_COLOR='red'

The colors should default to the above if the user doesn't set them.

NOTE: The explicit use of prefix RAFIKI is to avoid variable conflicts.