[O] Fontify emphasis markers

[Matthew Newton]

Hi Org folks,

I want to fontify emphasis markers differently than the actual emphasized text 
so as to de-emphasize (!) them. I noticed that `org-do-emphasis-faces` is 
responsible for hiding emphasis markers so I added a section like this to it:

(font-lock-prepend-text-property
 (match-end 4) (match-beginning 5) 'face 'org-emphasis-marker-face)
(font-lock-prepend-text-property
 (match-beginning 3) (match-end 3) 'face 'org-emphasis-marker-face)

I’m not wild about doing it this way since I have to redefine the entire 
`org-do-emphasis-faces` function in my own config.

Questions:
1. Was that the correct approach?
2. Does anyone else find this useful?
3. If useful, do you want a small patch?

Cheers,
Matt