[ESS] change font-lock-string-face for back ticks

[Munawar Cheema]

Dear ESS-help list:
I have a minor request:
It appears to me that function or variable names enclosed in back ticks 
get the font-lock-string-face. It is slightly irritating and it would be 
nice if this could be set to a different face. e.g.
In my emacs statements such as:
`%weirdly-named-function` <- function(){}
`var-name` <- "djsf"
The face of the function is the string-face and not the face for 
functions and similarly for the var-name face.

-- 
*Munawar Cheema *