ENH: Select numeric ExtensionDtypes with DataFrame.select_dtypes
#### Is your feature request related to a problem?

I would like to plot ExtensionArrays. DataFrame.select_dtypes(np.number) is called when plotting, which filters out ExtensionArrays. 

#### Describe the solution you'd like

DataFrame.select_dtypes(np.number) should select columns where ExtensionDtype._is_numeric is True, suggested here https://github.com/pandas-dev/pandas/issues/26173#issuecomment-503761441 

#### API breaking implications

Not considered.

#### Describe alternatives you've considered

Not considered.

#### Additional context

N/A