Rename and merge the blueprint-sortinghat command

[rcrowley]

A blueprint is currently a pretty monolithic thing, only really controllable via the blueprintignore file. This new tool provides a user-directed method of splitting one blueprint into two, perhaps a base and a customization.

[rcrowley]

It may go without saying, but this command has to be renamed before it actually gets merged.

[rcrowley]

Don't forget, this needs a manual page.

[rcrowley]

Matt and I discussed adding a blueprint-prune command that's the same as blueprint-split in every way except for the choice given to the user:

In blueprint-split the choice is between one blueprint and another.

In blueprint-prune the choice is between keeping a resource or not.

[rcrowley]

A practical note in favor of blueprint-prune over a two-argument form of blueprint-split: Python's optparse module makes it difficult to use any sort of placeholder for either dest-a or dest-b.