Forked and updated plugin

[gareth-gillman]

I have forked the plugin at https://github.com/gareth-gillman/cmb_field_map and extended it with:

• options page to add the api key
• on the meta box, checks if the api key is inputted

I am also going to hook into Google API key and check if the key is valid before saving to the DB (if I can).

Happy to pull request if Phil wants it in his plugin, using it on my own project so thought be useful to others since the google api change

[Mte90]

I see on https://github.com/mustardBees/cmb_field_map/pulls that there are more feature and a settings page can be not good for plugin developers instead a filter is better