add dyn_title to builtin.help_tags

[xz47sv]

Is your feature request related to a problem? Please describe.

When previewing help I'd like to know what file I am previewing, I think there should be a dynamic title option for help_tags.

Describe the solution you'd like

In my local copy I added:

    dyn_title = function(_, entry)
      return utils.path_tail(from_entry.path(entry, false, false))
    end,

to previewers.help in buffer_previewer.lua

Additional context

I didn't open a pull request, because I am unsure if it would be preferable for the filename to be a full path, just the tail or user configurable. But I can open one if my proposed solution is acceptable.