Support custom web components

[jeyj0]

Currently a view using custom components won't compile due to hsx not recognizing the tag, unless used with Text.Blaze.Html5.preEscapedToMarkup. This issue exist with the is attribute on existing elements as well.

Since custom components always need to have a hyphen in their name (<custom-component), we should be able to have a special case for them.

is should also be special-cased.

Maybe we should consider having a custmizable list of allowed custom components though, to add a bare minimum of checks for them.