Option to only follow new messages in file (--tail)

[timothyb89]

Largely applies to the kubernetes reader, but it would be nice to have some flag to skip the first chunk of input. Some kubernetes logfiles can get... unwieldy, and sifting through millions of old log lines is not a great use of time.

(woodchipper is still surprisingly quick at filtering through 7 million log lines, though, so at least there's that)

Vaguely related to #11