ENH: support coordinate access for indexing
`df.iloc[[0,1],[0,1]]` means return the square 0-1 rows and 0-1 columns

might be nice to have a way to mean return the _coordinates_ `(0,0),(0,1)`

related #7522

http://stackoverflow.com/questions/23686561/slice-a-pandas-dataframe-by-an-array-of-indices-and-column-names/23686855#23686855
