Limit and define available plugins

[obriencj]

Currently we directly install four plugins via requirements.txt

• pelican-image-process
• pelican-liquid-tags
• pelican-series
• pelican-yaml-metadata

We also clone the entire pelican-plugins repository and provide that (after removing three conflicting copies that we'd installed directly, earlier)

https://github.com/getpelican/pelican-plugins

We should add an explicit listing of what we want to pull from pelican-plugins, make it part of the container build process (via some additional file I suppose), and report those plugins clearly in the README.md

[obriencj]

We'll want a clearly defined set of plugins before v1 for certain.

[obriencj]

let's add these while we're add it:

• pelican-sitemap
• minchin.pelican.plugins.nojekyll
• summary (from git)
• extract_toc (from git)