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
The comments are pointless if the test has only three blank lines thus making the separation completely obvious. But sure :smile: