Interpret a single dash or slash as positional argument

[aivarsk]

Interpret a single dash as a positional argument which is a common convention for STDIN on Unix.

Fixes issue #57

[codecov[bot]]

Codecov Report

Merging #61 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #61   +/-   ##
=======================================
  Coverage   94.62%   94.62%           
=======================================
  Files           2        2           
  Lines         688      688           
=======================================
  Hits          651      651           
  Misses         37       37

Impacted Files	Coverage Δ
include/clara.hpp	93.17% <ø> (ø)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 47f772e...9314694. Read the comment docs.