Cover "implement a plugin manager"

[dtomvan]

See: https://github.com/sayanarijit/xplr/issues/443

Checklist:

• [x] The plugin manager will itself be a Lua plugin, to avoid touching the core.
• [x] An interface to manage the plugins interactively, without leaving xplr (with support for auto reload).
• [x] Pre and post install scripts.
• [ ] luarocks support.
• [x] fennel support.
• [x] Custom Lua path.
• [ ] Plugin load time benchmark.
• [x] Ignore plugins that fail to load.
• [x] Debug failures.
• [x] Dependency plugins.
• [ ] Dependency binary validation and installation support.
• [x] As declarative as possible.
• [ ] Display plugin readme.
• [ ] Plugin development utilities (watch, hot reload, inspection etc.)
• [x] Install from any git hosting platform.