BUG: parser can handle a common_format multi-column index (no row index cols), (GH4702)

TST: addtl mi malformed test cases

DOC: update io.rst docs for multi-index for columns
Merge pull request #5298 from jreback/mi_csv

BUG: parser can handle a common_format multi-column index (no row index cols), (GH4702)