Allow specifying arbitrary args for shellcheck

[ulope]

Unfortunately args currently only allows --enable.
This is problematic when other shellcheck options are necesarry.

Concrete use-case:
When sourcing files in scripts shellcheck requires the --external-sources / -x option to follow these. Otherwise it will fail with a SC1091 error.

Please enable passing all options through to shellcheck.

[LaurenceChau]

Hopefully it can allow to set severity

[aerickson]

I found this issue hoping to enable the following option in shellcheck.

-x --external-sources Allow 'source' outside of FILES

It would be great to be able to set this flag.