add a 'largo_top_term_class' function/filter thing to add the top term's class to the post_class

[benlk]

This would be really handy to have. Examples:

• A post with the top term set to the tag with the slug 'deep-dive' would have the class top-term-tag-deep-dive added
• A post with the top term set to the category 'finances-and-retributions' would have the class 'top-term-category-finances-and-retributions'

This would make it easier to style posts based on their top term.

https://codex.wordpress.org/Function_Reference/post_class

[benlk]

Example function: https://bitbucket.org/projectlargo/theme-voice-of-oc/src/e9cae82ab0a066025a4dea2cbe318a92db928520/functions.php?fileviewer=file-view-default#functions.php-350:370