Allow ShellCheck source= directive to be relative to project root instead of file path

[tbehling]

This PR introduces a new config setting for linter-shellcheck called useProjectCwd, which controls whether the paths used by ShellCheck's source= directive are relative to the project root or the file.

Background & Context

The ShellCheck tool supports a linter directive called source= that works like the following:

# shellcheck source=src/examples/config.sh
. "$(locate_config)"

In the current implementation of linter-shellcheck, the shellcheck command executes with a "current working directory" based on the file's path. The outcome is all source= paths must be relative to the file's directory.

While working with ShellCheck on a large project, I found it more convenient to reference absolute file paths from the source= directive. This is particularly helpful when running the shellcheck command from CI scripts. Without this ability, developers see different ShellCheck errors in Atom vs. CI results or a command line linter invocation.

To provide an option to homogenize the ShellCheck experience between Atom and typical CLI usage, this PR does the following:

• Introduces a new boolean config flag, useProjectCwd.
  • This defaults to false to maintain the existing behavior (file-relative paths).
  • Users can opt-in by checking this box to gain project-relative paths.
• Alters the "working directory" behavior when executing shellcheck to use either the project root or the file's directory.
• Adds specs and needed fixtures to exercise both the existing and new behavior.
• Updates the project README.

[Arcanemagus]

Minor change to how the configuration is specified, other than that this looks good!

[Arcanemagus]

This should probably be the description, although you may still want a title attribute to override the default of "Use Project Cwd" due to the "Cwd" part.

[tbehling]

Thanks for this feedback Landon, great point. I've updated the metadata and here's what it looks like now:

[Arcanemagus]

Thanks!