Add possibility to hook into modelgen plugin

[mskrip]

I realize there already is a branch modelgen-structtags for the mentioned issue and for what this change tries to accomplish. I'm opening a PR nonetheless at least as a suggestion since the feature has more general implications (possibility to mutate the whole generated models before rendering). Feel free to close this if the mentioned branch is close to solving the issue.

---

This change introduces option to implement custom hook for model
generation plugin without the need to completly copy the whole modelgen plugin.

One very possible case is as described in #876 and with this change the solution for
that can be:

func mutateHook(b *ModelBuild) *ModelBuild {
	for _, model := range b.Models {
		for _, field := range model.Fields {
			field.Tag += ` orm_binding:"` + model.Name + `.`  +  field.Name + `"`
		}
	}

	return b
}

...

func main() {
    p := modelgen.Plugin {
        MutateHook: mutateHook,
    }

    ...
}

I have:

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

[coveralls]

Coverage decreased (-0.04%) to 62.027% when pulling 2a269dd on mskrip:modelgen-hook into 99a55da on 99designs:master.

[JulienBreux]

Good job!

[unlinking]

Very useful feature for bson, mapstructure, camel to under score...

[vektah]

This needs some docs before we can

[vektah]

Those docs look great, thanks!