tsvPretty is the main loop, operating on input files and passing control to a TSVPrettyProccessor instance.
This separates physical I/O sources and sinks from the underlying processing algorithm, which operates on generic ranges. A lockingTextWriter is created and released on every input line. This has effect flushing standard output every line, desirable in command line tools.
This routine also handles identification of preamble lines. This is mostly for simplification of the TsvPrettyProcessor code.