addon commands

Seems only fair, that, like git runs git-annex, git-annex runs
git-annex-foo.

Implementation relies on O.forwardOptions, so that any options are passed
through to the addon program. Note that this includes options before the
subcommand, eg: git-annex -cx=y foo

Unfortunately, git-annex eats the --help/-h options.
This is because it uses O.hsubparser, which injects that option into each
subcommand. Seems like this should be possible to avoid somehow, to let
commands display their own --help, instead of the dummy one git-annex
displays.

The two step searching mirrors how git works, it makes finding
git-annex-foo fast when "git annex foo" is run, but will also support fuzzy
matching, once findAllAddonCommands gets implemented.

This commit was sponsored by Dr. Land Raider on Patreon.

doc/todo/allow_git-annex_addon_commands.mdwn

@@ -10,3 +10,5 @@ search the path for all "git-annex-" commands and then
 either dispatch the one matching the inputcmdname,
 or do autocorrect with the list of those commands 
 included along with the builtins. --[[Joey]] 
+
+> [[done]] --[[Joey]]