Give map-remove ability to remove multiple keys

 #1230

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Fix a reference to Sass::Util in exec/base.
Restore support for $key as map-remove keyword arg

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Add a changelog entry and some more docs for the new map-remove.
Merge branch 'map-remove-multiple-keys'

Closes #1230
Closes #1275