Common embedded templates

[natefinch]

Give users the ability to define templates that are parsed as embedded templates in the main schema enum and table templates.
This can be a great way to swap out content in templates so as to support multiple databases more easily. A good example is how postgres vs other databases specify value place holders. postgres wants $1 $2 etc and others want ? ? . if you have a "values" template that defines how to render that, then instead of copying the whole table template for each database type, you just need to swap out the values template.