Ability to add additional "link types" to the Link Fieldtype

[duncanmcclean]

Currently, the built-in Link Fieldtype only allows you to link to the following "things":

• URLs
• First Childs
• Entries
• Assets

However, there's a few situations where developers might want to add their own "link types" to the dropdown in the Link Fieldtype...

• Runway
  • In the context of Runway, it would be nice if you could select models between various models. (often people treat Eloquent Models like entries, so this is a fairly common request).
• Custom Asset Browsers
  • If you're integrating with a custom asset browser instead of using Statamic's native asset browser, you may wish to continue linking to assets via the Link Fieldtype.