ENH: data frame min/max and idxmin/idxmax
#### Is your feature request related to a problem?

I wish I could use an idxmax-like function to obtain the row and column coordinates of the minimum or maximum value.

#### Describe the solution you'd like

For a `DataFrame` object `df`, the proposed function `df.dfmin` would yield a tuple `(idx, col)` where `idx` would be the index of the minimum value (or a tuple of the hierarchical index's levels) and `col` the name of the column. For multiple equal values, a tuple of values could be returned.

#### API breaking implications

None.
