index: support --compact / -c on command-line

index: support --compact / -c on command-line

It's more convenient to specify `-c' / `--compact' on the
command-line when reindexing than it is to invoke
public-inbox-compact(1) separately.

This is especially convenient in low-space situations when
public-inbox-index is operating on multiple inboxes
sequentially, as compaction can happen immediately after
indexing each inbox, instead of waiting until all inboxes are
indexed.