* fix(cat): do not cat directories
Fixes#707
* fix(head): do not let head() read directories
Also fixes a typo
* fix(sort): do not sort directories
Also fixes a typo
* fix(tail): do not let tail() read directories
Also fixes a typo
* fix(uniq): do not let uniq() read directories
We also had a test which called sort() instead of uniq(), so we never
actually tested the missing-file case. This fixes that as well.
This also throws an error for using a directory as output.
* fix(pipe): fix breakages with piped commands