fix #1237 remove unreachable types from generated code

[vikstrous]

Rebased and improved #1238

Left to do:

• Tests for various edge cases. Maybe just make a big file full of unreachable types? https://github.com/99designs/gqlgen/pull/1238/files#diff-41df4b43d528566617cd5b0c3ae5adc68f75a5c2d4fe8a5bb3a5ad10ccad9508
• Figure out the right way to handle federation (then Entity type)
• Look into removing unused directives
• Figure out how to report / not report the warning. Maybe use the "verbose" flag for this? Maybe extend the verbose flag to "log level"?

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage increased (+0.3%) to 74.918% when pulling ae8064c on vikstrous:unreachable2 into 33fe0b9 on 99designs:master.