Make models configurable via template

[penard-cb]

Similarly as in #2720 and #2725 this PR makes the models configurable as well via template. This opens up a lot of new options for configuration and using the templating functionality

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 79.291% (+3.6%) from 75.694% when pulling d6a15b0 on penard-cb:feat/ConfigurableModelsTemplate into abe3ffd on 99designs:master.

[StevenACoffman]

This is pretty neat! I use sqlc + gqlgen, and quite frequently the models are duplicative, but you cannot easily translate from one struct type to the other. Using mapstructure requires you to add a tedious amount of yaml config for gqlgen so that the structs will be annotated properly.

type User struct {
    Username string `mapstructure:"user"`
}