Allow for redefinition of template syntax per-file

[dclark@…]

The Genshi syntax of using $text or ${text} is exactly the same syntax used by the bourne shell (bash sh etc) and close to that used by Make and the Makefile.

So if you want to for example template a Makefile that includes bourne shell code, you are going to end up having to do a lot of ugly stuff like $$$$ - see also #282.

It would be great if the syntax that Genshi uses for text templates could be redefined per-file like it can be for example with Cheetah ( #compiler-settings). This would allow the user to choose syntax that does not conflict with the type of document the user is templating.