Add support for shellcheck source-path=SCRIPTDIR

[mickaelperrin]

Hi,

I see that some work has been made to have a partial support of the source-path directive in #141.

However, I am unable to have it working as explained in (https://github.com/koalaman/shellcheck/wiki/Directive#source-path) the shellcheck documentation:

#!/bin/sh
# shellcheck source-path=SCRIPTDIR
here="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
. "$here/config.sh"
. "$here/utils.sh"

In that example, both sourced files would trigger the BashSupport Pro can't follow a non-constant source. error.

Hope it will be introduced soon 👍