Rewrite elements with hyphenated tags as blocks

`<x-foo src="{{src}}"></x-foo>` becomes `{{#x-foo src=src}}{{/x-foo}}`
`<x-foo src="{{upcase src}}"></x-foo>` becomes `{{#x-foo src=(upcase src)}}{{/x-foo}}`
`<x-foo src="a{{b}}c{{d e}}"></x-foo>` becomes `{{#x-foo src=(concat "a" b "c" (d e))}}{{/x-foo}}`