Add ability to resolve all TagHelperDescriptors with one method call.

- Modified the AddOrRemoveTagHelperSpanVisitor to no longer manage TagHelperDescriptors found in the system. Instead it now manages TagHelperDirectiveDescriptors which are then used to resolve TagHelperDescriptors.
- Changed the signature of ITagHelperDescriptorResolver to take in a TagHelperResolutionContext which will allow us to pass more information without breaking tooling.
- TagHelperDescriptorResolver now resolves all TagHelperDescriptors at once and manages descriptors found in the system based on values on the provided TagHelperDirectiveDescriptors.

#214
Modify tests to use new ITagHelperDescriptorResolver signature.

- Updated all of the tests to use the new ITagHelperDescriptorResolver signature so instead of passing strings they now construct TagHelperDescriptorResolutionContexts.
- Removed several tests from the AddOrRemoveTagHelperSpanVisitorTests. This was due to the change in responsibility of managing the found TagHelperDescriptors; the TagHelperDescriptorResolver now does this.
- Added several new tests to verify the TagHelperDescriptorResolver manages resolved TagHelperDescriptors based on the given TagHelperDirectiveDescriptors.

#214
Add hook to add TagHelperDirectiveDescriptors.

- Also added a test to validate that you can hook into the TagHelperDescriptorResolutionContext.

#214
Changed TagHelper attributes to be SpanKind.Code if not string typed.

- Also added tests to validate that non string TagHelper attributes inherit the SpanKind.COde behavior.
- Removed a block wrapping around single markup spans.