renamed TagNameAttribute to HtmlElementNameAttribute
Change TagHelper tag and attribute names to be lower snake cased.

- Lower snake cased casing is the HTML convention.

#240
Modify tests to account for new tag and attribute names for TagHelpers.

- Also added additional tests to validate new lower snake casing.

#240
Fixed naming from snake => Html case.

- Essentially from pascal/camel case to lower kebab-case.