Add config file for setting default placeholder values

[cmalven]

It's becoming pretty tiring having to change the default support@craftcms and http://craftcms.com placeholder values for every single new Linkit field I create.

Would love to have the ability to define these default values in a linkit.php config file or in a CP setting.

[jsunsawyer]

This would go along nicely with my suggestion here: https://github.com/fruitstudios/craft-linkit/issues/31