configure shellcheckrc

This removes so many annoyances:

```bash
% cat ~/.shellcheckrc
# ShellCheck: Global Rules
#------------------------------------------------------------------------------
# Disable these annoying rules
disable=SC2317,SC2154,SC2155,SC2086

# Allow the sourcing of files
external-sources=true
```

REF: https://gist.github.com/todd-dsm/1616b791455a3be5d2abf3718f6c8ef1