Feature request: Add ability to override template per entry

[kaidesu]

Is your feature request related to a problem? Please describe.

Individual entries should have the ability to override and specify a custom template to load in place of the default.

Feature request checklist

• [x] This is a single feature (i.e. not a re-write of the entire CMS)
• [x] The title starts with "Feature request: " and is followed by a clear feature name (Ex: Feature request: add konami code to control panel)
• [x] I have searched for existing issues and to the best of my knowledge this is not a duplicate