Ignore baseline errors

[williamjallen]

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

Adding ShellCheck to existing projects can be difficult due to a large volume of existing errors which are impractical to fix at one time. Would it be possible to add some sort of "baseline" ignore option similar to the way PHPStan does it? I have looked through the documentation quite a bit but the best current solution I can find is to simply turn off errors for some files, or add a large number of inline ignores, which is not ideal when there are several thousand errors to be resolved.