[lagru]

Adds a plugin system based on pluggy. For now, we only allow plugins as standalone Python files that must be explicitly loaded in the configuration.

Closes #114, since this PR makes expansions like requested in #114 possible (see this plugin example).

Also includes some performance improvements (caching of doctypes per worker)
and a good amount of refactoring to make this possible. 😮‍💨

Release note

For maintainers and optionally contributors, please refer to changelist's README on how to document this PR for the release notes.

Add a new plugin system to Docstub with 2 hooks.
These hooks allow overriding/extending the default grammar and 
transformer that are used to transform doctypes into annotations.
Add a minimal guide and documentation too.
{label="enhancement"}

Cache transformed doctypes in each worker.
Aggregated cache statistics are printed when showing debug
information (`-vv` option).
{label="performance"}