WIP: Added TagHelperContext.UniqueId

- The ID is created at view compilation time and is unique per TagHelperExecutionContext and thus per HTML element in the source for which Tag Helpers will run
- #241
- Still need to update tests, may need to add some plugability to do it well, e.g. ITagHelperUniqueIdGenerator
`{ get; }` 

We just started using the feature hence why the rest of the file still has private sets. If you want to change the rest, feel free, if not, no biggie :smile: .