Add support for custom rulesets to Inso

Hello! I'd like to extend inso with the concept of custom rulesets. It looks as though [linting is performed by Spectral]( and with that in mind I'd like to expose Spectral's ability to [set custom rulesets]( through inso. I considered putting the ruleset in the .insorc file initially, but this would not allow us to have different rules per API when working with legacy systems. I looked in to how Insomnia stores the API specs as I intended to put the rulesets alongside the specification, but it looks like they're stored in a database. ~~To keep things easy, I'm proposing an ~/.insomnia/rulesets folder that will contain rulesets. Running inso lint spec <file> --ruleset my-custom-ruleset would load ~/.insomnia/rulesets/my-custom-ruleset.yml. This does not follow the $XDG_CONFIG_HOME rules which would have me store the rulesets in ~/Library/Application Support/Insomnia/rulesets, but is more expected for consumers imo.~~ (This won't work due to sy…