Clean up claims except for select claims

[belikor]

This follows from the closed issue #6, and #7.

At the moment the generic function space.cleanup_space looks at all claims. The parameter never_delete considers channels to avoid when deleting claims.

Maybe a new parameter keep could be used to avoid deleting select claims, either by 'name' or by 'claim_id', regardless of the channel.

keep = ["is-justin-bieber-a-christian-singer:d",
        "b17e56e5f3b476b7f7a82916a340028aa9292f87"]
dd = t.cleanup_space(main_dir="/opt", size=1000, percent=90,
                     keep=keep, what="media")