Add log-run mode that submits given files to Logstash and prints the results.

[cresh]

Log-run mode is useful if you want to pass example log files through your filter
and take a look at the result. Log-run mode makes this easy by using your test
configuration so that you do not have to modify your pipeline configuration or
maintain a development pipeline configuration.

The command-line argument --logfile=LOGFILE enables log-run mode and passes
LOGFILE to Logstash and prints the result of your pipeline to stdout. You can
provide command-line argument --logfile LOGFILE multiple times. If LOGFILE
is a directory, log-run mode finds all the log files that match
--logfile-extension below the given directory and processes them (default
logfile extension is log).

[cresh]

I've rebased this pull request on top of current head. It is still very useful to me, maybe you want to consider accepting it?