[FEA] Implement skipinitialspace read_csv parameter

This parameter is already a part of the API, but it is not used internally.

From experiments with Pandas, it looks like this parameter is only relevant when determining if a given field has a special value (True, False, NaN). With `skipinitialspace=True`, leading spaces should be ignored and files with content like `" NaN\n n/a\n"` can then be parsed correctly.