allow transforming query text before passing it to `graphql-language-service-interface`

I'd like to be able to append to editor's query text (or more generally transform it) before sending it to `graphql-language-service-interface`.

## Motivation

In https://github.com/gatsbyjs/gatsby we allow supplying fragments that users can use freely in their queries (we transform queries and attach used fragment definitions before query execution with the help of `relay-compiler`). This works well in queries to users write in their source code, but doesn't work currently in graphiql because those 3rd party fragments are not in document and `codemirror-graphql/lint` report errors (missing fragment definitoins) and `codemirror-graphql/hint` also can't provide autocompletion hints for those fragments it doesn't know about.

## Proposed design

Allow passing extra option to addons - for example `transformQueryText` function (that will default to identity transform if not specified), that will be exucuted before passing query text to `graphql-language-service-interface` functions (`getDiagnostics` for lint / `getAutocompleteSuggestions` for hint):
```
CodeMirror.fromTextArea(myTextarea, {
mode: 'graphql',
lint: {
schema: myGraphQLSchema,
transformQueryText: queryText => doSomethingWithQueryText(queryText)
},
hintOptions: {
schema: myGraphQLSchema,
transformQueryText: queryText => doSomethingWithQueryText(queryText)
}
});
```

## Example of this in action:

In here I make use of fragments that are not in document, but are supplied by Gatsby, and both lint (no "unknown fragment" error) and hint (autocompletion for fragments supplied by Gatsby) works as expected for Gatsby usecase:

Above screenshot is result of some monkey patching (just to to make proof of concept that this can work).

I would be happy to submit pull request, but wanted to check first if this is something that would be welcome here.