Initial commit of map operator

This looks full of escaping errors, but it’s working on some toy
examples.
Check for null response to a map command
Rename (since variable is not specific to gists!)
Oops! This shouldn’t be here…
Add header row handling to map; tidy up a bit
Minor tweak for consistency
Add some basic docs for map op
Merge pull request #105 from andylolz/21-map-op

Map operator