Support OpenAPI v3

**DocFX Version Used**:
2.35.4

**Template used**:
`default`

**Steps to Reproduce**:

1. Clone [docfx-seed](https://github.com/docascode/docfx-seed)
2. Add [petstore.yaml](https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/petstore.yaml) to cloned repo inside `restapi/`
3. Add yamlmime `### YamlMime:ManagedReference` in the first line of `restapi/petstore.yaml`
4. Build with docfx

**Expected Behavior**:
* Successful build
* Working output with documented REST API
* Use of yaml files as is (without yamlmime)
* Support of OpenAPI v3

**Actual Behavior**:
```
Build failed.
[18-05-08 12:55:54.098]Error:System.AggregateException: Mindestens ein Fehler ist aufgetreten. ---> Microsoft.DocAsCode.Build.Engine.Incrementals.BuildCacheException: Full build hasn't loaded model restapi/petstore.yaml
bei Microsoft.DocAsCode.Build.Engine.HostService.<>c__DisplayClass94_1.b__1()
bei Microsoft.DocAsCode.Build.Engine.Incrementals.IncrementalUtility.RetryIO(Action action)
bei System.Threading.Tasks.Parallel.<>c__DisplayClass42_0`2.b__1()
bei System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask)
bei System.Threading.Tasks.Task.<>c__DisplayClass176_0.b__0(Object )
--- Ende der internen Ausnahmestapelüberwachung ---
bei System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
bei System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
bei System.Threading.Tasks.Parallel.PartitionerForEachWorker[TSource,TLocal](Partitioner`1 source, ParallelOptions parallelOptions, Action`1 simpleBody, Action`2 bodyWithState, Action`3 bodyWithStateAndIndex, Func`4 bodyWithStateAndLocal, Func`5 bodyWithEverything, Func`1 localInit, Action`1 localFinally)
bei System.Threading.Tasks.Parallel.ForEachWorker[TSource,TLocal](IEnumerable`1 source, ParallelOptions parallelOptions, Action`1 body, Action`2 bodyWithState, Action`3 bodyWithStateAndIndex, Func`4 bodyWithStateAndLocal, Func`5 bodyWithEverything, Func`1 localInit, Action`1 localFinally)
bei System.Threading.Tasks.Parallel.ForEach[TSource](IEnumerable`1 source, ParallelOptions parallelOptions, Action`1 body)
bei Microsoft.DocAsCode.Build.Engine.HostService.SaveIntermediateModel(IncrementalBuildContext incrementalContext)
bei Microsoft.DocAsCode.Build.Engine.CompilePhaseHandlerWithIncremental.PostHandle(List`1 hostServices)
bei Microsoft.DocAsCode.Build.Engine.PhaseProcessor.Process(List`1 hostServices, Int32 maxParallelism)
bei Microsoft.DocAsCode.Build.Engine.SingleDocumentBuilder.BuildCore(PhaseProcessor phaseProcessor, List`1 hostServices, DocumentBuildContext context)
bei Microsoft.DocAsCode.Build.Engine.SingleDocumentBuilder.BuildCore(DocumentBuildParameters parameters)
bei Microsoft.DocAsCode.Build.Engine.SingleDocumentBuilder.Build(DocumentBuildParameters parameters)
bei Microsoft.DocAsCode.Build.Engine.DocumentBuilder.BuildCore(DocumentBuildParameters parameter, IMarkdownServiceProvider markdownServiceProvider, BuildInfo currentBuildInfo, BuildInfo lastBuildInfo)
bei Microsoft.DocAsCode.Build.Engine.DocumentBuilder.Build(IList`1 parameters, String outputDirectory)
bei Microsoft.DocAsCode.SubCommands.DocumentBuilderWrapper.BuildDocument(BuildJsonConfig config, TemplateManager templateManager, String baseDirectory, String outputDirectory, String pluginDirectory, String templateDirectory)
bei Microsoft.DocAsCode.SubCommands.BuildCommand.BuildDocument(String baseDirectory, String outputDirectory)
bei Microsoft.DocAsCode.SubCommands.BuildCommand.Exec(SubCommandRunningContext context)
bei Microsoft.DocAsCode.SubCommands.CompositeCommand.Exec(SubCommandRunningContext context)
bei Microsoft.DocAsCode.Program.ExecSubCommand(String[] args)
---> (Interne Ausnahme #0) Microsoft.DocAsCode.Build.Engine.Incrementals.BuildCacheException: Full build hasn't loaded model restapi/petstore.yaml
bei Microsoft.DocAsCode.Build.Engine.HostService.<>c__DisplayClass94_1.b__1()
bei Microsoft.DocAsCode.Build.Engine.Incrementals.IncrementalUtility.RetryIO(Action action)
bei System.Threading.Tasks.Parallel.<>c__DisplayClass42_0`2.b__1()
bei System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask)
bei System.Threading.Tasks.Task.<>c__DisplayClass176_0.b__0(Object )<---
```