Change parameter checks and other fixes

This commits contains the following modifications:

- Change parameter checks from number of parameters ($#) to content length of
  each parameter ( -z "$1" ).
  A command called with an empty string in the first parameter *still* receives
  1 parameter ($# = 1), while checking the string length will work in both
  cases.

- Change commands that may be aliased to use the builtin / non-modified
  command, using the `command' prefix (e.g. `git' -> `command git').

- Changed the initialization sequence of the plugin, from using a function
  to being inline in the script.
  It also uses the $0 parameter which is set when the script is sourced, and
  it contains the script filepath.
  Example: with command `source test.sh' -> in test.sh, $0 will be `test.sh'.

- Various style fixes, including missing space and some comments.