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