Handle mutlipart/form-data

[koblas]

This PR should support Multipart form-data file upload requests as specified by https://github.com/jaydenseric/graphql-multipart-request-spec -- which is adopted by the Apollo team. It also works without file uploads as a general handler.

[coveralls]

Coverage increased (+2.6%) to 89.231% when pulling 5f3718a on koblas:master into 59006c0 on graphql-go:master.

[koblas]

@chris-ramon Nudge - would it be possible to have this looked at?

[duong-se]

hi all, When will it merge to master ?

[koblas]

@chris-ramon Any chance that this could get a review, will clean up the conflict shortly.

[appleboy]

@koblas conflicts

[koblas]

@appleboy Conflicts resolved... Hopefully, someday @chris-ramon will feel like merging.

[danjenson]

Can we merge this?

[hongbo-miao]

While waiting this being merged, found an alternative handler supporting mutlipart/form-data created by @asif-mahmud.

Originally posted at graphql-go/graphql#607
Library is at https://github.com/asif-mahmud/gingraphql

Just provide one more option 😃