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
Reacting to System.Threading version changes
Add line mappings to project TagHelper attribute values.

- We now create LineMappings for instances where a TagHelper's attribute value is not of a string type.
- This will enable the Razor editor to create projections from .cshtml => .cs for TagHelper attributes.
- Modified the TagHelperCodeGenerator and TagHelperBlockBuiler to accurately track the attribute values start locations so it could flow into code generation.
- Modified existing tests to account for the new line mappings.

#207
Addressed code review comments.
Remove ContentBehavior.

- Removed all tests and instances of ContentBehavior in preparation for moving to a non-ContentBehavior based design.
- Removed ContentBehavior specific CodeGeneration.

#221
Re-design TagHelperOutput and runtime dependencies to allow all content modes.

- Added PreContent, PostContent and ContentSet properties to TagHelperOutput.
- Added GeneratePreContent, GeneratePostContent and SupressOutput methods to TagHelperOutput.
- Added multile ExecuteChildContentAsync and GetChildContentAsync to the rendering phase, ultimately only exposing GetChildContentAsync to a TagHelper author.
- Added more knowledge of StartWritingScope and EndWritingScope to the TagHelper runtime components. This is to enable the runtime components to utilize the RazorPage's infrastructure to render a delegate to a writer and retrieve its value to ultimately expose it to the user.

#221
Modify tests to allow new content mode design for runtime components.

- Added a new internal ctor for TagHelperExecutionContext since it's used in multiple tests to allow for less friction testing.

#221
Added CodeGen for TagHelper content mode redesign.

- Modified the CSharpTagHelperCodeRenderer to understand a single line of TagHelper rendering (instead of doing different things based on ContentBehavior).
- Modified existing CodeGen output to reflect new content changes.

#221
Rebased and addressed code review comments.