Add more flexible custom tag naming

[kostacoffee]

Currently, the regex for htmlTagName requires a custom tag to have a dash - in it. This is not necessary since custom tags can be written without a dash.

A good regex match would allow for removal of all htmlTagName literals and instead allow the regex to do the work.

[othree]

- is required

https://www.w3.org/TR/custom-elements/#prod-potentialcustomelementname

[othree]

Current regexp is incorrect. I will fix it later.

[kostacoffee]

Made a start on this in #80. Just need to add the special characters now.

[othree]

80 is not ok.

You can send another PR if you have a new regexp match the spec.