and a general option for this in the settings.

There is already a default map zoom in the settings in version 2.1

Yes, there is a default zoom, is it possible to override that zoom level?  On my
site, I've got a map on each post, zoomed to streetlevel (13), but on the category
page, I want to see all of the mapped items I've got, a zoom level of 5 might be more
appropriate for that one.

I definitely agree this is very important

Please try to provide custom zoom levels in next release

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Next release won't include the ability to do this through shortcode - a little more javascript fiddling to get that to 
work. However, I intend to add an option to set the zoom when setting the location for a post.