Add option to watch/tail a single file

[mgar]

You might want to choose just one single file to stream logs instead of streaming them all to stdout, so we decided to add a new flag to filter by filename:

rdstail --instance omsusstg tail -n 10 -f trace/staging_database_w000_28520.trc

rdstail --instance omsusstg watch --file trace/alert_staging_database.log