archrelease: Validate tags before releasing

Compare every single tag with a list of valid tags. This prevents broken
releases which occurred whenever someone made a typo on the command
line:

    $ ./archrelease community i686
    ==> ERROR: archrelease: Invalid tag: "community" (use -f to force release)

Since the list is used in the ZSH completion as well, break it out to a
separate file and move it to "lib/". Also, add a command line parameter
to allow for releasing to an unknown repository when necessary.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>