Add 6to5 support & better errors for processors

Includes server side support via https://github.com/jsbin/pennyworth/pull/36 and includes error handling via our linting tool (gutter, footer and otherwise).

I've also updated the output renderer, so instead of not showing anything in the output when pennyworth fails to compile, but now shows the error in the source.

For instance, this is an error in the 6to5 server side compile echoed back to the full output page:

```html
<script>
Line: 3
Character: 55
Message: unknown: Unexpected token (4:55)
  2  | // 2
  3  | var foo = 'bar';
> 4  | var [a, [b, c, d] = ["hello" [", ", "junk"], ["world"]];
     |                                                       ^
  5  | console.log(a + b + c); // hello, world
  6  |
</script>
```