Added include guard again

Now also checking if compatible has been set
Made mapping configurable

The mapping is now called in autoload, which gives the user the chance
of changing the default textobjext mapping from 'a' to something else
Put most of the stuff in autoload

The mappings would not work because I forgot to move the functions they
were referring to with them
Added docs
Use canonical <Plug>(argtextobj...) mappings to allow remapping.

Using the argtextobj#Enable() autoload function is counterproductive, as it triggers the autoload function on VimEnter, negating the startup speedup of autoload.
It is more natural and flexible to use the standard <Plug>-mappings instead of a configuration variable to influence the mappings.
Use proper syntax for help tag definitions.