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
Part of: https://github.com/aspnet/Razor/pull/243/files#r20884427

Undo changes to this file (no need for config anymore).