Added CLI option to pass the path to shinken.ini as parameter.

Hi,

the default ~/.shinken.ini is fine when calling the script interactively.

But when using with a config management tools (eg. Puppet, Chef) or through a script, the user it is executed as might not be consistent.
It those cases, passing the path to shinken.ini as an argument allows to make sure the proper config is being used and avoid warnings.

This pull request allows it.
The code is a bit of a hack, but since the script itself has a disclaimer saying it's ugly... ;)