colour: Make alpha a feature

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
m4: Get the DocBook conditions variable from NK_INIT

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
token: Reorder some code

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
token/tests: Rename to the full namespace

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
token: Use a refcount-based freeing

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
token: Go back to a constant string as the callback return value

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
token: Add enum-based tokens

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
token: Add support for after/before string on tokens

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
token: Do not copy useless empty strings

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>