Add support for arbitrary conflict resolutions

[olsen232]

$ sno conflicts -s --flat
table_A:fid=5
table_A:fid=6
table_A:fid=7

$ sno resolve table_A:fid=5 --with-file=5.geojson
Resolved 1 conflict. 2 conflicts to go.

The user can provide an arbitrary number of features in a geojson file, and store them as the resolution to a conflict. Generally, the appropriate number of features will be either 1 - in the case of an edit/edit conflict that needs manual resolution - or 2, in the case of an add/add conflict, the resolution should probably contain both features in some form. (Zero is also a useful number of features, but it is already possible to resolve a conflict by supplying zero features without providing a file).