Add ability to validate Link and Embedded url

[maxpeterson]

Thanks for providing a great library @jpuri

I needed the ability to validate the links so I have created this PR to add config options to validate the urls input for Link and Embedded

• Add linkValidator config options to provide functions to validate Link target
• Adds valid/invalid classes to Link modal title and target
• Disable Add Link button when not target valid
• Add embedValidator config options to provide functions to validate Embedded url
• Adds valid/invalid classes to Embedded modal link, width and height
• Disable Add Embedded button when link not valid

This is partly based on #355, however it does not provide "allowRelative" link functionality nor prepends links with 'https://'. Both should be possible with a combination of the new linkValidator and the existing linkCallback

Possibly related to #483