Split commands into independent hooks.

This prevents commands from hanging when called from ssh.
Also use `pluginhook -s` to run commands.
Add a simple interactive shell to run dokku commands.
Allow the logs' `-t` option to come before or after the app name.

Also fix a problem with detaching from the container.
Add a few utility commands.

A `docker command which (obviously) runs docker, and an `ls`command
which lists the deployed applications.
Prefix all commands in plugins with the plugin's name.

Don't use pluginhook to run commands.
Renamed 00_dokku-standard into core.
Add support for aliases.
Change the help command to be suited to the new command format.

Every user-facing command should contain a special help message after its
shebang.
The help command parses it, and display the correct command, based on the
ALIAS file.
Include a default set of aliases.
Use DOKKU_ORIGINAL_COMMAND in usage messages.
Merge git-* into a single git:shell command, using aliases.
Allow printing help for a single plugin.
Use an underscore as the default command.
Add an alias plugin to manage the ALIAS file.
Export a PLUGIN_ROOT variable.
Move the "shell" command into its own plugin.
Use an inputrc to disable (useless) file completion in the shell.
Make 'test_deploy' use the short version of the url command.