Embed templates into source

[clayne11]

What happened?

Using this library with Bazel is currently challenging for a number of reasons. One is the usage of packages.Load. The other is that the template files are not included in the binary.

genqlient recently made this change and not only is the API much nicer, it is much easier to consume with Bazel.

Are you open to a similar PR for gqlgen?

[frederikhors]

Please PR. Thanks.

[jamesthompson]

@clayne11 This would be very very welcome from me and I'm sure a bunch of others as well. Thanks a lot in advance if you're able to work on this!