Add TagName attribute to specify tag names for Tag Helpers.

- Made it so the TagHelperDescriptorResolver could pull tag name targets from TagNameAttribute's on types.
- Tested the behavior between TagNameAttribute and TagHelperDescriptorResolver (the only consumer of the attribute).
- Made sure to test duplicate and inherited class scenarios.

#120 

We could throw upon finding duplicates here.  I thought a non-throw approach might be better though.