Add `get_by_slug`

This filter would allow you to do a reverse lookup based on the slug of the page you are looking for:

```
# custom Jinja2 filter
def get_by_slug(objs, slug):
    for obj in objs:
        if obj.slug == slug:
            return obj

JINJA_FILTERS = {
    "get_by_slug": get_by_slug,
}
```
https://siongui.github.io/2017/01/08/pelican-get-single-page-or-article-by-slug-metadata-in-theme/