Respect COMPOSER_HOME environment variable

Resolves #5
Fix local variables in completion hooks using global scope
Make completions run for the `composer global` command

Resolves #2
Support completion on global command shortcut 'composer g'
Add command name completion for `composer help`

Resolves #3
Fix assumption that array index is defined
Add `remove` and `update` completions based on required packages

Resolves #4
Fix double command name suggestion when using completing for global/g