Move attributes to preprocess function to allow for theme override

Not sure if $vars['settings'] is the best place for these since that consists of actual settings from the display formatter. I can move them to $vars['attributes'] or something similar if that would make more sense.

Updated to use $vars['attributes'] to store values to keep them separate from $vars['settings'] that holds actual UI settings.

Actually, I'm going to go even further here and remove the theme function entirely and replace it with a render array generated in the preprocess function and a template file. That will give themers full control of the output and will align better with best practices. I'll do some work on that and probably have a patch by mid-week.

Patch is attached. It's a fairly significant rework of how the default field formatter and the theming layer interact. It will allow for near complete freedom in theming the HTML and CSS through adding preprocess functions and overriding the template files. Assuming I didn't make any mistakes, it should also, by default, render identically to how the current module renders, so it will be backwards compatible. Since I changed the theme render names, a cache clear will be required for fields to display after updating. That happens automatically when update.php is run, so I don't think that will be a big issue.

The only people who may have an issue with the update are those who overrode the theme function in a custom module or theme to get custom output. They will see the field revert back to default styles. Overriding the theme function is a really terrible practice, so while I wish it wouldn't do that to them, I don't see another way around it.

As to D8, I'm not currently running any D8 sites, but most of the D8 theming conventions are just more strongly enforced versions of the D7 best practices. I actually took a lot of inspiration (and code) from the D7 and D8 paragraphs module and the core D8 image module. Everything I've done here should transfer seamlessly to D8 with a reworking of the template files to use twig. I may have some time to put into that in the next month. Let me know if there are any issues or changes needed, and hopefully others will test it and it will work well.

Fixed some incorrect documentation.

I'm working on a D8 patch, but it will likely be another week or two before I can get it finalized.

I'd like to get some eyes on the D7 version to make sure it's all working as expected before I commit a lot of work to porting this to D8.