add an option to choose file extension

[svengau]

According the graphql-spec, .graphql is the official extension. It would be great if we could have an option to choose our extension, with default to .gql to maintain compatibility. If it sounds good for you, I can push a PR.

ps: Thanks for the good job on this really useful plugin.

[timqian]

Thanks @svengau, this option sounds useful, PR is welcomed

[svengau]

Hey @timqian here is the PR #48
thanks