Add ability to search for comment strings

New option -C for searching for the comment string in the header.
To make things easier I've refactored out the the way to get a
pointer for the right bit of the record into its own function
Added in a test for finding records using comments

Currently the test fails, not because it fails to find a record
but because the print function adds in an extra space between the
identifier and the comment. I'll fix this later