Add text alignment

[petebytes]

Thank you very much for making this repo available to others!

This PR adds left, center and right text alignment.

Feel free to make any changes you deem necessary.

[petebytes]

Found an issue :(
When I save it looks correct
Stored in DB
<h1><div style="text-align: center;">Headline</div></h1>

but when Trix loads after saving this is the result
<h1><div style="text-align: right;"><div><div>Headline</div></div></div></h1>

[petebytes]

updated the last comment to show code

[petebytes]

On the commit I listed evaicons - the icons are actually from https://material.io/resources/icons/?style=baseline