Add omit_complexity config option for issue #2502

[mariuscraciunescu]

Full description in issue #2502

Introduced a new config flag omit_complexity, which when set to true will skip the generation of the body of the following:

type ComplexityRoot struct {
  // empty struct
}

func (e *executableSchema) Complexity(typeName, field string, childComplexity int, rawArgs map[string]interface{}) (int, bool) {
  ec := executionContext{nil, e}
  _ = ec

  // switch statement is not generated

  return 0, false
}

Fixes the following compiler issue when GraphQL schema is too large:
<autogenerated>:1: internal compiler error: NewBulk too big: nbit=13244 count=1469951 nword=414 size=608559714

I have:

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

[coveralls]

Coverage: 75.636%. Remained the same when pulling 3bd9dce on mariuscraciunescu:omit_complexity_option into f5764a8 on 99designs:master.

[StevenACoffman]

Thanks!