Enhancement: Add `generate` command, replaces `manpage` and `autocomplete`

The `generate` command has been added, which replaces the now removed
commands `manpage` and `autocomplete`. This release of restic contains the
most recent manpages in `doc/man` and the auto-completion files for bash and
zsh in `doc/bash-completion.sh` and `doc/zsh-completion.zsh`

https://github.com/restic/restic/issues/1274
https://github.com/restic/restic/pull/1282
