Make `graphql-config` peer / optional dependency?

[dotansimha]

At the moment it's installed anyway, which might install a lot of other sub-deps.

[dimaMachina]

Otherwise we need to install:

• @graphql-tools/graphql-file-loader
• @graphql-tools/code-file-loader
• @graphql-tools/load
• @graphql-tools/url-loader
• @graphql-tools/json-file-loader

All of them already included in graphql-config, so we simple install graphql-config and use him to load schema and operations from graphql-config config file or via parserOptions.
I made a refactoring In this PR #545

[dimaMachina]

@dotansimha As an idea, we can reduce the bundle by publishing GraphQLRuleTester as own package @graphql-eslint/rule-tester because only rule tester uses @babel/code-frame https://github.com/dotansimha/graphql-eslint/blob/master/packages/plugin/package.json#L37

[dimaMachina]

this dependency is required even for programmatic usage, but in browser we don't use it (since there is no fs here)

I would like still keep it as dependency