Add `strings` module from Go for template processing

Currently, the `TEMPLATE` parameter in the `Modelfile` is a simple Go template. For example, I can do something like this to print first 25 characters of the model's response: ```go {{ printf "%.*s" 25 .Response }} ``` However, this basic processing is not usable. What I'm trying to do is to remove the BOS token from the model's output. The token is `<|begin_of_text|>` and it is outputted not in every message, but at the beginning of the dialog. One possible solution is to use Go's `strings` module to strip the prefix: ```go {{ .Response | strings.TrimPrefix <|begin_of_text|> }} ``` but this is not supported in ollama. Can we include extra functions for template processing?