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
Make a test helper method to reduce noise in this file: 
`TagHelperChunk CreateTagHelperChunk(string tagName, IEnumerable<TagHelperDescriptor> tagHelperDescriptors)`