Add a -f, --force global switch (#435)

Add a -f, --force global switch (#435)

This global modifier is available in Alire.Force. Other candidate locations,
like Alire.Config, Alire.Errors, Alire.Utils.User_Input cannot be used because
they bring in non-preelaborable units and it snowballs from there.

The recommended usage is through Alire.Recoverable_Error, which will warn or
raise appropriately depending on the flag. The boolean Alire.Force is also
available for less straightforward situations (interaction defaults for
example).