Add the proper config bin directory to `PATH` instead of the previously (incorrect) fixed `~/.composer/vendor/bin`. Nowadays the right config dir is `~/.config/composer/vendor/bin`.
This is because we can't be sure which arguments require a json file
and which don't, so the `else' statement will always be difficult to
check for correctness.
Use $_comp_command1 for command list generation instead of 'composer':
`$_comp_command1` contains the command written in the command line.
This allows us to have a different composer name
This plugin currently doesn't autocomplete composer commands if there is no composer.json in the current dir. However, the commands create-project, init, search, selfupdate and show are still useful without a composer.json!
