benchtests: Enable scripts/plot_strings.py to read stdin

This patch enables scripts/plot_strings.py to read a benchmark result
file from stdin.
To keep backward compatibility, that is to keep accepting multiple of
benchmark result files in argument, blank argument doesn't mean stdin,
but '-' does.
Therefore nargs parameter of ArgumentParser.add_argument() method is
not changed to '?', but keep '+'.

ex:
  $ jq '.' bench-memset.out | plot_strings.py -
  $ jq '.' bench-memset.out | plot_strings.py - bench-memset-large.out
  $ plot_strings.py bench-memset.out bench-memset-large.out

error ex:
  $ jq '.' bench-memset.out | plot_strings.py

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>