Liara

* Introduce a new plugin system. This moves the `has-pending-document` command into a new plugin and adds signals to interact with liara's processing.
* Remove `liara.version.version`. Use the standard `liara.__version__` variable instead, which was already present in earlier versions.

* Add a `server_rule_only` option to prevent the creation of redirection nodes and use the redirection paths verbatim.

* Add `has-pending-document` to the command line. This will check if there is any content which is filtered by the `liara.site.DateFilter`. This is useful for cron-based deploys which try to not rebuild if there are no content changes, as there is no other way to find out if all content in a given revision has been published.

* Fix `liara create-config` not working.

* Add `liara.cache.Sqlite3Cache`, which allows caching everything into a single file instead of one file per entry.
* Add a bytecode cache for the `liara.template.Jinja2TemplateRepository`.
* Fix generated nodes not getting generated when using `liara serve`.
* Reduce debug spew when stopping `liara serve` using `^C`.

* Add `liara.template.SiteTemplateProxy.get_page_by_url`.