template-fragments for jinja-like engines

Usage with minijinja (see also examples/minijinja.rs):

```rust use templatefragments::{joinpath, split_templates};

let mut source = minijinja::Source::new();

for (path, template) in [ ("index.html", includestr!("templates/index.html")), ("users.html", includestr!("templates/users.html")), ] { for (fragmentname, templatefragment) in splittemplates(template)? { source.addtemplate(joinpath(path, &fragmentname), &template_fragment)?; } } ```