text/template: add a slice function to the predefined global functions 

I'm suggesting to add a function named `slice` to the predefined global functions under the `text/template` package that will mimic the slice expression in Go. 

Calling `slice` returns the result of slicing its first argument by the following arguments. 
Thus `{{ slice x 1 3 }}` is, in Go syntax, x[1:3]. Each sliced item must be a string, slice, or array.
