api: Add get_stream_email_address().

[rsashank]

Python wrapper function for the new endpoint introduced in ZFL 226 to fetch a stream's email address. This could enhance the ease of making calls within zulip terminal and potentially other clients.

[neiljp]

@timabbott LGTM based on manual test, and given lack of old-server fallbacks and inconsistent nearby styling to compare to (see below).

It would be good to get this out, unless more API calls would be beneficial to add based on new changes/features in 7.5+/8.0?

It looks like we don't note/handle lack of features in older servers in zulip, ie. on an older server this call would simply fail with this call - the only use of feature_level is for method parameters, not the presence of the feature.

Reviewing adjacent code, we're not particularly consistent with style:

• absence/presence of responses to examples
• regular/unicode strings in example responses (eg. u'result': u'success' vs dropping u)
• example layout (blank lines, or no lines, from 'Example usage' to code)
• historically I thought we had call_endpoint calls split over lines (possibly just black?)