feat:goctl model mongo add easy flag for easy declare.

[czyt]

with -e ,we will auto generate collection Name and no need a param in Model creation,for example:
create UserModel without e

// NewUserModel returns a model for the mongo.
func NewUserModel(url, db, collection string) UserModel {
	conn := mon.MustNewModel(url, db, collection)
	return &customUserModel{
		defaultUserModel: newDefaultUserModel(conn),
	}
}

create UserModel with e:

const UserCollectionName = "user"
// NewUserModel returns a model for the mongo.
func NewUserModel(url, db string) UserModel {
	conn := mon.MustNewModel(url, db, UserCollectionName)
	return &customUserModel{
		defaultUserModel: newDefaultUserModel(conn),
	}
}

[kesonan]

LGTM, It's an awesome pull request, thanks you!