Add 'before' hook

[bertpareyn]

This pull request adds a before hook that allows a function to be executed before initialising the edit form. before defaults to false so no changes are required for existing code.

$('.jeditable-field').editable(editableSubmitted, {
    'before': function() {
        // Do things before the edit form is initialised
    },
});

[nicolaasmatthijs]

@tuupola : Any chance that this can be merged?

[NicolasCARPi]

Hello,

Thanks for your contribution, the code has been merged.