Add support for missing attributes

[jonkoops]

Currently the x-select component supports a limited amount of the available attributes on a normal HTML select. The attributes that are currently not present on the component are as follows:

• autofocus
• form
• required
• size

For a complete overview of the available attributes and their functionality see: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/select#attributes

Some of these attributes will probably not be applicable to most Ember applications (mostly form and name), but they should probably be added as well to support some edge cases. Other attributes like required and autofocus are obviously pretty useful.

I can whip up a pull request to add these attributes, but I wanted to do a quick check if anyone has issues with adding these attributes.

[cowboyd]

A pull request would be very much appreciated! There is #10 which is outstanding, but I think it makes sense to roll them all into one PR.