allow schemaPath to come from package.json

[gagoar]

👋 Thanks for creating graphql-schema-linter.

We are using it to lint our schemas and we would like to have everything (when possible) in our package.json, particularly we noticed that SchemaPath can only be provided via the command line or programatically but not via the configuration.

this PR provides schemaPaths as part of the configuration.

[gagoar]

in this last commit I've done a couple of things:

• Moved schemaPaths test to package_json/test.js
• Added test for customRulePaths given that the functionality was already there but was missing the test. ( during testing I've added the same functionality than schemaPaths given that the paths are relative to the configuration as well).
• Added comments to the Readme.md for all the cosmic options that can be provided.
• Flipped a couple of assertions given that assert helper shows errors exepcting assert.equal(expected, actual)