Allow arbitrary HTML for the options with custom elements

[jachin]

Instead of using the <select> tag to setup the options have another API "path" that would make use of custom elements. This would allow just about any markup in the options (and the groups).

So things could look something like:

<much-select>
  <much-select-option option-value="fun" slot="much-select-option-fun">
   <h1>Hurray!</h1>
   <p>So much fun</p>
  </much-select-option>
  <much-select-option option-value="bear" slot="much-select-option-bear">
   <h2>Another one</h2>
   <img src="a/picture.jpg">
  </much-select-option>
</much-select>

I think things will work out better if each option has it's own unique slot.

[jachin]

It might be better to do this instead: #250

[jachin]

To finish this feature there are some remaining issues to work though:

In the selected value part of input field, what do we show in single select and in multi select? Do we just show the select value? Or a label?

When we're filtering down the options by typing, what text is the filter working off of? Does
(should) this feature even work if a <much-select>'s options are being rendered this way?