allow merging of projects

[propensive]

The fury universe projects diff command should, for each difference, give each difference a unique identifier. We could then provide a fury universe projects merge command which imports a change from one project to another.

For example,

fury universe projects merge -p magnolia@6fba88 -a magnolia@6437c9 -m aFZ z98 fB0 cXd

where the three short hashes at the end are identifiers for the differences between the two projects, and would be listed in the diff table.