Implement templating

[Zebouski]

I think the scope of https://github.com/leewillis77/wp-github-oembed/pull/25 was too grand, they were trying to implement new styles. They also only covered only one of the four functions, this commit covers them all

[leewillis77]

Hi;

Thanks for submitting this. I'll review it this week and let you know any comments!

[leewillis77]

Thanks for this. I've merged this with a few changes:

• <?php echo instead of <?= (personal preference)
• Renamed template files, and look for them in a plugin-named folder in the theme

Thanks for your contribution.