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
So the tests follow the same format as the others in the project, add `// Arrange`, `// Act` and `// Assert` comments to the corresponding unit tests. Ex for the first one:

```C#
[Fact]
public void CreatesAUniqueIdForSingleTagHelperChunk()
{
	// Arrange
	var chunk = new TagHelperChunk
	{
		TagName = "div",
		Descriptors = new[] { new TagHelperDescriptor("div", "DivTagHelper", "FakeAssemblyName", ContentBehavior.None) },
		Children = new List<Chunk>(),
		Attributes = new Dictionary<string, Chunk>()
	};
	var codeRenderer = CreateCodeRenderer();

	// Act
	codeRenderer.RenderTagHelper(chunk);

	// Assert
	Assert.Equal(1, codeRenderer.GenerateUniqueIdCount);
}
```