factor page content processing out of parse_xrefs

Factors this code out of parse_xrefs() where it never belonged, into a new function process_page_content() that is called from main after the main parse has succeeded and only if text extraction was requested, i.e. -x or -X was passed on the command line.

Fixes #49 (closed).

Also, as a tangential change, makes it so that -x and -X always create (and truncate) their respective output files. I feel that is more likely to be what the user expects. If there is no text, the requested output file should be empty after the program ran, rather than untouched.