Plain text ingestion improvements/tasks

[nblumhardt]

From #19

• [ ] Extend --invalid-data= to include report as an option, so that "orphan"/un-parseable lines can be captured
• [ ] --test flag that prints JSON to STDOUT instead of sending events to Seq (could perhaps be called something like --echo or --dry-run?)
• [ ] List the built-in matchers in the README
• [ ] Sanity check the (future) story for optional ({:?), parameterized (digit(4)) and regex matchers