Added a few functions.

usage will show a message by either specifying a second argument
when calling the function or filling out $__USAGE_ERR.

arg now has an argument called error.
It does not die as it should when calling exit.
We'll have to keep messing with this.
Added a new string function and a new test.

__APPEND now exists in the string library.

Alias have also been created for the single underlined string
functions.  I have yet to get rid of them.  (Whenever I feel
like updating my code.)