ENH: add mode.mangle_dupe_cols option GH3468
#3468

as discussed, an added option to disable th mangling behavior
of dupe columns taken by pd.read_csv going back to olden days.

Not sure what the default should be, currently the default is compatible
with existing scripts.

Tested with both c and python parser engines.

what about doing this as an argument to `read_csv`, say `duplicate_column_names=False`, default should prob be `False` but that breaks API ......
 yeah, I'll go for that. still not sure about the default though. Still compatible right now.
if it breaks back compat, I'll push this to 0.12.
 actually I think we should put in the option now
but leave the default = True so doesn't break API compat
(then maybe break in 0.12)?
 works for me, added note to RELEASE.rst.
