Adding support for custom delimiters. 

This is extremely useful if you are using grunt-templates to generate JSP (Java) or ERB (Ruby/Rails) code (which is my case). I just added a minor modifications to the task file so that it also accepts a property called 'delimiters' wich is what `grunt.template.process` accepts as an optinal parameter (http://gruntjs.com/api/grunt.template#grunt.template.process).

I tried to take care of the docs too, I hope this can be merged into master!

If you find anything, please let me know.