Change parameter checks and other style fixes

This commits contains the following modifications:

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

- 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.
Merge pull request #7 from mcornella/omz-bootstrap

Change parameter checks and other style fixes