Add TagHelperRunner for TagHelper runtime.

- The TagHelperRunner is responsible for calling into the process methods of the TagHelpers.
- This involved adding the following core classes: TagHelper, TagHelperExecutionContext, TagHelperOutput, TagHelperContext.  All of which aid in running TagHelpers.
- Tested TagHelperRunner, TagHelperOutput and TagHelperExecutionContext.

#154
- `TagHelperContext` doesn't exist and the class name isn't necessary anyhow.  so just `<see cref="AllAttributes"/>`.
- nit, bit wordy. suggest `/// Tracks the HTML attribute in <see cref="AllAttributes"/>` here and `/// Tracks the HTML attribute in <see cref="AllAttributes"/> and <see cref="HTMLAttributes"/>` at line 61.  "Adds the HTML attribute to" also works.

FYI can add
```
#xml-docs-test
  k-xml-docs-test
```
to Razor's makefile.shade and execute `.\build.cmd xml-docs-test` to find broken XML documentation.  Good news is the main assembly only has 6 broken docs at the moment.  aim to keep that number the same (unless you happen to be changing one of the existing offenses, then fix it).
would be nice to include `target='test'` in that makefile.shade addition.  then will check the docs whenever `.\build.cmd` runs tests.  of course this isn't an immediate issue because it requires clean docs.