ENH: Allow opting in to new dtypes on I/O routines via keyword to I/O routines
With the new dtypes (`IntegerArray`, `StringArray`, etc.), if you want to use them when reading in data, you have to specify the types for all of the columns.  It would be nice to have the option to use the new dtypes for all columns as a keyword to `read_csv()`, `read_excel()`, etc.

(ref. discussion in pandas dev meeting on 11/20/19)
