.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