added Seek() function to FileReader

Adds a new function to the FileReader implementation of the
Reader interface for seeking within a file. It additionally
adds a new Status code to represent that a request to use
the Seek() function has failed, which will happen when the
file is stdin or something similar.

Change-Id: Iae299a3b36a8440da941b182c8865429ee3515d4
4 files changed