conform with pep8's preferences on explicit line continuation

I just wrote this and I don't know anything about vimscript, so I'd let me run it a few days before merging it.

But it seems to fix the stuff pep8 hates about the way vim-python-pep8-indent currently would format this file:

    from worst.module.ever.written import \
            some_crazy_long_function_name_we_imported, \
            wierd_loop_with_a_crazy_long_explicit_break, \
            pep8_wants_multiple_of_four_indent, and_we_do_not_do_that
    
    
    bad_global_var_gets_set_to_the_result_of = \
            some_crazy_long_function_name_we_imported()
    
    
    for x in wierd_loop_with_a_crazy_long_explicit_break + \
             pep8_wants_multiple_of_four_indent:
        and_we_do_not_do_that()


... and I wanted to go ahead and throw it out there tho so I don't forget.  If you don't hear back - it's probably fine!

Assuming everything works out this probably fixes issue #10