offer 'parents' option: make containing directories if necessary/possible

[gojomo]

It's be nice if there were a smart_open(..., parents=True) parameter, which (for local files) would create parent directories, if necessary – much like mkdir --parents.

[tmylk]

Should it be like this?

[gojomo]

Yes, that seems the right approach to handling the dir-making – of course it'd need to be provided the base-directory for the target file path.

[menshikh-iv]

I close this PR, reason: #169 (comment).