Add httpdynamic examples

[motiejus]

This example shows how we can generate GraphQL schema on runtime.

Credit to @kormoc for the dynamic reloading part.

[coveralls]

Coverage decreased (-0.8%) to 82.652% when pulling 52aaf3c on Motiejus:httpdynamic into 22050ee on graphql-go:master.

[coveralls]

Coverage decreased (-0.8%) to 82.652% when pulling 36a6dd6 on Motiejus:httpdynamic into 22050ee on graphql-go:master.

[coveralls]

Coverage decreased (-0.8%) to 82.695% when pulling 8452576 on Motiejus:httpdynamic into a77f106 on graphql-go:master.

[chris-ramon]

Thanks a lot @motiejus 👍

LGTM 🚢