Add Plugin Feature to the graphiql IDE

[kburson]

Fixes #.

Changes proposed in this pull request:

Add a plugin feature to the graphql playground web ide.

At T-Mobile we are using Graphql apollo servers and wish to make use of the Graphiql IDE for testing and demonstration purposes. Our backend data servers use a security protocol known as POP signatures, RFC 7800. In order to use Graphiql we needed to add this token binding functionality. The best way was to extend the IDE to sign every request sent. For our solution we needed 2 plugins, one to initialize the security keys and pop token, and a second to sign each request as it was sent.

The changes to the graphql-playground enable a generic plugin structure that is passed in during instantiation of the server.

We have included a readme document on how to use the plugin feature, how to enable and configure the plugins when instantiating your apollo server. The actual functions used for the plugins are completely up to the user, given a simple api pattern to follow. These functions need to be hosted by a cdn or the apollo server as they will be downloaded to the browser when the playground is initialized in the window.

If you need more details, please let me know.

Thanks.

[apollo-cla]

@kburson: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/