Add new CLI subcommand for inferring intersphinx mappings

[bskinn]

Most of the time, what's planned in #149 for the CLI will suffice -- a separate CLI interface just for mapping inference is YAGNI.

For the cases like Django, with a nonstandard objects.inv location, the standard inference has no hope of succeeding, because the objects.inv URL passed to sphobjinv suggest will not align with the uri of any objects in the Inventory.

The API should work to infer a mapping in these cases, but some users might want a CLI.