BUG/ENH: Warn when falling back to python parser

-raise ValueError when engine='c' specified with unsupported options
-raise ValueError when fallback to python causes options to be ignored
-produce ParserWarning on fallback to python when no options ignored
-fix bug in C parser with leading whitespace and \r-delimited files
 (add test)
-translate sep='\s+' to delim_whitespace=True (add test)
-specify engine='python' in tests with sep='\s+' and multiindex column
 input (work around GH 6893)
-add 'engine' option to docstring of read_csv and read_table
-copy tests that previously fell back to python from ParserTests to
 TestPythonParser and check that they raise ValueErrors when run
 under other engines