TreeChildren and TreeParent type function option as eg ManyToOne and OneToMany

[nolazybits]

Issue type:

[ ] question
[ ] bug report
[x] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[ ] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[x] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:
I'n using DI everywhere in my application and the types are resolved at runtime.
ManyToOne and OneToMany decorators have a type function options, like this for instance

@ManyToOne((ty: typeof IFamCategory) => StaticContainer.Get(IFamCategory), (category: IFamCategory) => category.childCategories)
public abstract parentCategory: IFamCategory;

it would be nice to have the same for the TreeChildren and TreeParent decorator like

@TreeChildren((ty: typeof IFamCategory) => StaticContainer.Get(IFamCategory))
public abstract childCategories: IFamCategory[];

@TreeParent((ty: typeof IFamCategory) => StaticContainer.Get(IFamCategory))
public abstract parentCategory: IFamCategory;

Thanks again for this awesome library.