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]

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)	⬆️

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