Add possibility to hook into modelgen plugin

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:

```golang
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,
    }

    ...
}

```

plugin/modelgen/models.go

@@ -11,6 +11,12 @@ import (
 	"github.com/vektah/gqlparser/ast"
 )
 
+type BuildMutateHook = func(b *ModelBuild) *ModelBuild
+
+func defaultBuildMutateHook(b *ModelBuild) *ModelBuild {
+	return b
+}
+
 type ModelBuild struct {
 	PackageName string
 	Interfaces  []*Interface
@@ -50,10 +56,14 @@ type EnumValue struct {
 }
 
 func New() plugin.Plugin {
-	return &Plugin{}
+	return &Plugin{
+		MutateHook: defaultBuildMutateHook,
+	}
 }
 
-type Plugin struct{}
+type Plugin struct {
+	MutateHook BuildMutateHook
+}
 
 var _ plugin.ConfigMutator = &Plugin{}
 
@@ -221,6 +231,10 @@ func (m *Plugin) MutateConfig(cfg *config.Config) error {
 		return nil
 	}
 
+	if m.MutateHook != nil {
+		b = m.MutateHook(b)
+	}
+
 	return templates.Render(templates.Options{
 		PackageName:     cfg.Model.Package,
 		Filename:        cfg.Model.Filename,

plugin/modelgen/models_test.go

@@ -1,6 +1,7 @@
 package modelgen
 
 import (
+	"go/ast"
 	"go/parser"
 	"go/token"
 	"io/ioutil"
@@ -14,7 +15,9 @@ import (
 func TestModelGeneration(t *testing.T) {
 	cfg, err := config.LoadConfig("testdata/gqlgen.yml")
 	require.NoError(t, err)
-	p := Plugin{}
+	p := Plugin{
+		MutateHook: mutateHook,
+	}
 	require.NoError(t, p.MutateConfig(cfg))
 
 	require.True(t, cfg.Models.UserDefined("MissingTypeNotNull"))
@@ -42,4 +45,30 @@ func TestModelGeneration(t *testing.T) {
 			require.True(t, len(words) > 1, "expected description %q to have more than one word", text)
 		}
 	})
+
+	t.Run("tags are applied", func(t *testing.T) {
+		node, err := parser.ParseFile(token.NewFileSet(), "./out/generated.go", nil, 0)
+		require.NoError(t, err)
+		for _, obj := range node.Scope.Objects {
+			if spec, ok := (obj.Decl).(*ast.TypeSpec); ok {
+				if st, ok := (spec.Type).(*ast.StructType); ok {
+					for _, field := range st.Fields.List {
+						fieldName := strings.ToLower(field.Names[0].String())
+						expectedTag := "`json:\"" + fieldName + "\" database:\"" + spec.Name.String() + fieldName + "\"`"
+						require.True(t, field.Tag.Value == expectedTag)
+					}
+				}
+			}
+		}
+	})
+}
+
+func mutateHook(b *ModelBuild) *ModelBuild {
+	for _, model := range b.Models {
+		for _, field := range model.Fields {
+			field.Tag += ` database:"` + model.Name + field.Name + `"`
+		}
+	}
+
+	return b
 }