Replace \0 by \g<0> in python sub

> The back reference \g<0> substitutes in the entire substring
> matched by the RE.
> http://docs.python.org/2/library/re.html#re.sub

The documentation does not mention \0 as an alias to it.
Although \1 to \9 works.

I guess this alias (\0 <=> \g<0>) existed in old python version,
but it is not the case any more.

In python 2.7 and 3.3:
> import re; re.compile(r'123').sub(r'@\0@',"ab123cd")
'ab@\x00@cd' # KO
> import re; re.compile(r'123').sub(r'@\g<0>@',"ab123cd")
'ab@123@cd' # OK
Merge pull request #78: Bug fix for accelerated highlighting using Python