support --stdin-filename when passing files via stdin

[vapier]

For new checks and feature suggestions

• [x] https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• [x] I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/sh
[[ $# -eq 0 ]]

Here's what shellcheck currently says:

$ shellcheck test.sh 

In test.sh line 2:
[[ $# -eq 0 ]]
^------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

For more information:
  https://www.shellcheck.net/wiki/SC3010 -- In POSIX sh, [[ ]] is undefined.

$ shellcheck - <test.sh

In - line 2:
[[ $# -eq 0 ]]
^------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

For more information:
  https://www.shellcheck.net/wiki/SC3010 -- In POSIX sh, [[ ]] is undefined.

Here's what I wanted or expected to see:

The output of - in the diagnostics is confusing, especially when passing other files. It's also problematic when using --external-sources as SCRIPTDIR isn't set to anything useful.

Would like to have an option like --stdin-filename so that I can pass test.sh and it'll use that name when processing stdin.

[j0sh]

This feature would be useful. Right now I have to annotate stdin checks separately to distinguish them from one another.

[Ordoviz]

This would be useful when hacking together a simple shellcheck wrapper where parsing shellcheck's JSON output is not worth it. For example, in markdown-shellcheck I have to use regexes to replace the - with the filename.