.shellcheckrc: enable optional checks

* Suggest explicitly using -n in `[ $var ]`. https://www.shellcheck.net/wiki/SC2243 & https://www.shellcheck.net/wiki/SC2244
* Suggest 'command -v' instead of 'which'. https://www.shellcheck.net/wiki/SC2230
* Suggest quoting variables without metacharacters. https://www.shellcheck.net/wiki/SC2248
* Require [[ and warn about [ in Bash/Ksh. https://www.shellcheck.net/wiki/SC2292

Author: nl6720

.shellcheckrc

@@ -0,0 +1,11 @@
+# Suggest explicitly using -n in `[ $var ]`
+enable=avoid-nullary-conditions
+
+# Suggest 'command -v' instead of 'which'
+enable=deprecate-which
+
+# Suggest quoting variables without metacharacters
+enable=quote-safe-variables
+
+# Require [[ and warn about [ in Bash/Ksh
+enable=require-double-brackets