feat(import): allow configuring custom root fields

[nfantone]

This PR stems from #42.

Allow extending the currently internal rootFields array with a custom mergeableTypes array.

const schema = importSchema('./schema.graphql', { mergeableTypes: ['Foo'] });

Types declared in mergeableTypes will be treated as root fields, just like Query, Mutation and Subscription. This effectively means:

• They can be imported as Foo.*.
• Fields across all imported files will be merged together in a single type.
• Will appear at the top of the resulting SDL.

BREAKING CHANGE: importSchema API changed. It no longer receives schemas as its second, optional argument. Instead, it should be defined as a property in the new options argument.

const schemas = { appSchema, postsSchema, commentsSchema }

// Before
const schema = importSchema('./schema.graphql', schemas)

// Now
const schema = importSchema('./schema.graphql', { schemas })

[nfantone]

I'm open to discussing how the new, optional mergeableTypes argument should be passed to importSchema.

• A BREAKING CHANGE can be avoided by either making options a third argument (cumbersome if we don't really care about providing schemas) or by exporting a new function -in addition to importSchema-.
• mergeableTypes could also be named something different, like customRootFields or additionalRootFields, more in line with vocabulary currently used by the documentation.

[ardatan]

Your changes has been merged into graphql-toolkit. Thank you!