This adds the following command:

    $ swh alter remove --dry-run https://example.com/swh/graph2
    Removing the following origins:
     - swh:1:ori:8f50d3f60eae370ddbf85c86219c55108a350165
    We would remove 11 objects:
     - swh:1:ori:8f50d3f60eae370ddbf85c86219c55108a350165
     - swh:1:snp:0000000000000000000000000000000000000022
     - swh:1:rel:0000000000000000000000000000000000000021
     - swh:1:rev:0000000000000000000000000000000000000018
     - swh:1:rev:0000000000000000000000000000000000000013
     - swh:1:dir:0000000000000000000000000000000000000017
     - swh:1:dir:0000000000000000000000000000000000000012
     - swh:1:dir:0000000000000000000000000000000000000016
     - swh:1:cnt:0000000000000000000000000000000000000014
     - swh:1:cnt:0000000000000000000000000000000000000011
     - swh:1:cnt:0000000000000000000000000000000000000015

The flags `--output-inventory-subgraph`, `--output-removable-subgraph`,
and `--output-pruned-removable-subgraph` can be used to produce various
Graphviz `.dot` source files, that can produce a visual representation
with the `dot` command.

Actual removal will come at a later time. Creating records
of soon-to-be-removed object should be implemented before that.

Based on the algorithm and code designed by olasd. Thanks vlorentz
and olasd for the help and reviews.

Close #1