Add initial EditorConfig support

[ralish]

Projects may define shfmt settings as part of an .editorconfig file. Where these settings are present we probably want to use them so shell formatting is consistent with the project settings.

This commit introduces a new setting to enable usage of settings defined in an .editorconfig file: useEditorConfig (boolean). The default is false to preserve existing behaviour.

Enabling this setting does three things:

1. Disables addition of any custom shfmt flags defined in the flag setting. If any custom flags are provided to shfmt it will ignore .editorconfig settings, making this setting mutually exclusive with the new setting.
2. Disables addition of the -i (indent) flag for the same reason as above.
3. Launches shfmt in the directory of the file we're formatting. This ensures shfmt is able to locate any .editorconfig file in the project (which may be further up the directory hierarchy).

[ralish]

This PR is flawed. It works in my configuration, but won't in many other cases due to the way shfmt is called. I'll see if I can make it a bit more durable to handle these other typical cases.

[ralish]

OK, new commit pushed which reworks my original approach of simply invoking shfmt in the same directory as the file being formatted. That worked on my setup because the shfmt settings in my .editorconfig were configured to apply to all files (i.e. [*]). It won't work for settings scoped to specific paths or extensions as the content to format is passed in via stdin.

New approach instead retrieves the EditorConfig configuration for the specific file to be formatted. It does so by introducing a dependency on the editorconfig package, with which it retrieves the configuration for the file via the parseSync() function. It then sets up the appropriate shfmt flags before invocation. All of this still only occurs if the user has enabled the useEditorConfig setting.

This commit also introduces a broader refactoring of the formatDocumentWithContent() function. It fixes a few miscellaneous issues and I think is now cleaner and easier to read. Hope that's OK!

[ralish]

Any chance this could be merged? I've just rebased it with all the recent commits :)

[ralish]

Thank-you for merging! Looking forward to this being in the next release :-)