Stable version 0.6.0

This release is dedicated to Factorio. If this is how much fun it is to
build factories and oppress natives, then history makes a lot of sense.

CHANGELOG.md

-## ???
+## v0.6.0 - 2018-12-02
 ### Added
 - Command line option --severity/-S for filtering by minimum severity
 - Command line option --wiki-link-count/-W for showing wiki links

ShellCheck.cabal

 Name:             ShellCheck
-Version:          0.5.0
+Version:          0.6.0
 Synopsis:         Shell script analysis tool
 License:          GPL-3
 License-file:     LICENSE