removing Jinja2 dependency

`ggg` is intended to be a system-wide tool. Being a user of such tool, I don't really want to keep Jinja2 installed for my system python.

We just use `string.replace` for now.

P.S. I was thinking of `string.Template` first. However, you'd have to replace all values from `$PATH` to `$$PATH` and that is retarded.