Add a nib initializer to avoid hard coded nib name

[JuyeonYu]

This is one of inevitable hard coded string cases for nib. -> ReusableCell<MyCell>(nibName: "MyCell")

In this pull request, we can init it without hard coded string anymore. -> ReusableCell<MyCell>(nibType: MyCell.self)

That will be ok on condition of same string between nib and class name.