Allow to specify additional compendiums

[mondeja]

If you want to use mkdocs-mdpo-plugin to translate the documentation along with mdpo pre-commit hooks to translate a README, you could end translating the README file if it is included in the documentation with mkdocs-include-markdown-plugin. So add a config setting named compendiums to pass custom compendiums for the translations, allowing to pass the README.es.po file created with mdpo pre-comit-hooks to the mkdocs-mdpo-plugin translation process.