JS helper functions; Dynamic URLs

• Use JavaScript helper function inside the templates, e.g.
  {{ .Routes | fn `routes => routes.filter(r => r.Meta.tags?.includes("some_tag"))` }}
• Use create multiple routes from a single source file by using a variable name in the source filename, e.g. [tag].html and providing values for that variable in the front matter, e.g.

---
tag: ["a", "b", "c"]
---

Skip loading partials when no partials found

• Fix #2

Use partials without route templates

• Add templating even when a route does not specify a template. This allows the user to use partials and template pipelines without specifying a template.

Initial release

This is a pre-release to make testing tinystatic in practice easier, YMMV.