Removed old unused code.
Relocated human-friendly std(in|out|err) path strings to the io
namespace.
Changed the unclaimed argument allowing to provide help description for
the unclaimed arguments.
Added io::InputStream and io::OutputStream wrappers to provide
abstractions over standard I/O streams and tty's. Removed the need for
OpIStream and OpOStream. Allowed command-line tools to more easily accept
multiple input file arguments. Fixes #3.
Abstracted out the isatty check in make_stdout_pager to use the
io::OutputStream helper.