Switch CLI API dispatch framework to `docopt_dispatch`

[guyzmo]

The current command dispatch code has been designed to avoid an over-complex main function. But it was after coding it that I discovered that the author of docopt implemented a similar framework he called docopt_dispatch.

That framework might need some adjustments for custom features built in the current command dispatch code, but it's worth using an existing and common framework than reinventing the wheel.

[pyhedgehog]

Any docopt-alike implementation is very fast for development, but it has limitations that can't be fixed:
You can't allow to implement addiotional commands/options. This will block plugin author from adding service-specific commands (i.e. for service with locally-accessible repositories something like git --git-dir=/home/git/gogs-repositories/user/repo.git remote ...).

[guyzmo]

well, this is a more global design issue, and this discussion better fits in #28 …

This issue is only there as a reminder to replace my custom command "dispatch" with docopt_dispatch, because I basically reinvented the wheel without knowing.