Add Config support for specifying a PlaygroundEndpoint

[beaspider]

In some deployments e.g. serverless the default endpoint being set for the Playground does not work. Have added a PlaygroundEndpoint field to the Config to allow this to be explicitly specified (if this is not provided the default will still be used)

[coveralls]

Coverage increased (+0.2%) to 87.685% when pulling 3157880f2716aca6277b97f193bcf4113ed6abd9 on homemade:master into c1267356e6539d975e83bbea383c2bbb861dce5b on graphql-go:master.