Function name consistency

[k5cents]

The tidyverse style guide recomends verbs for function names.

As well as following the general advice for object names, strive to use verbs for function names:

The gluedown functions are inconsistent:

1. Some functions use the proper verb format: e.g., md_define()
2. Others use ambiguous tense: e.g., md_bold(), md_list(), md_quote()
3. And some use the improper noun format: e.g., md_heading()

[k5cents]

I think the verb (?) md_head() just doesn't make sense. I'm switching back to md_heading(). Most other functions are well named.