Support a transforming in the indexer

[Matt Godbolt]

In issue #12, it became clear that being able to transform the index before storing it would be very useful. For example, to make case consistent.

[Matt Godbolt]

5aab30b may go some way to start this process. Now there's a -p/--pipe command so:

$ zindex ~/audit.log.gz -p 'jq --raw-output --unbuffered .eventId'

works, for example.