BUG: inconsistent and undocumented option "converters" to read_excel

Issue #8212 (first part): pandas.read_excel accepts an optional argument "converters"  (which is passed down to PythonParser) to convert single cells in columns with a conversion function. I documented this feature and added a try/except block to make it work in case some cells contain NaNs.

What's still missing is the full "dtype" argument, a la read_csv. That patch is somewhat orthogonal because it only works with the C parser, so I plan to implement it in a second step.