Closes#26.
Acts like --count but emits only the paths of files with matches,
suitable for piping to xargs. Both mmap and no-mmap searches terminate
after the first match is found. Documentation updated and tests added.
For example, when only a single file (or stdin) is being searched, then we
should be able to print directly to the terminal instead of intermediate
buffers. (The buffers are only necessary for parallelism.)
Closes#4.
