feat: --silent will silence error output

[nfischer]

Fixes #61

This works with anything supported by shell.config. So if you want to change shell.config.foo to the value true for the ls command, you can specify shx --foo ls.

shx --silent ls won't silence all output, only error output, since that's currently how config.silent works. If we want shx to work differently, I can modify this to do a special check for the --silent flag and also silence stdout.

[codecov-io]

Current coverage is 90.62%

Merging #62 into master will increase coverage by 0.30%

@@             master        #62   diff @@
==========================================
  Files             4          4          
  Lines            31         32     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits             28         29     +1   
  Misses            3          3          
  Partials          0          0          

Powered by Codecov. Last updated by 3041d6c...70b3e14

[ariporad]

LGTM!