Add support for HTML linting / validation at build time

[thescientist13]

A nice feature of JSX / React / ESLint was that if an HTML tag was missing it's closing tag (for example) a linting error could be raised, ex.

 @ ./.cache/sync-requires.js 30:67-185

 WAIT  Compiling...                                                                                                                 10:17:16

 ERROR  Failed to compile with 1 errors                                                                                             10:17:17

 error  in ./src/pages/blog/2018/06/01/index.jsx

Syntax Error: Expected corresponding JSX closing tag for <p> (24:8)

  22 |             we would be happy to have you join the convesation<p>
  23 |
> 24 |         </div>
     |         ^
  25 |       </BlogPost>
  26 |     );
  27 |   }

 @ ./.cache/sync-requires.js 30:67-185

Opened a corresponding issue over in lit-html

[thescientist13]

moved to polymer/tools