Make eslintRulesDir relative to working directory

If 'eslintRulesDir' is set, attempt to find it relative to the project root.

This little change makes it possible to have project-specific rulesets, as the rule directory doesn't need to be absolute (although that's perfectly fine too).

Without this, setting the value to e.g. "eslint-rules" would typically search in directory of the currently edited file, and result in an error such as "Error: ENOENT, no such file or directory '...myProject/app/views/MyView/eslint-rules'".