One of the biggest releases in a while!

The main theme for this release is **experimental support for gem docsets**.

This required significant changes on all fronts - both back-end and client (TUI).

There is now a "POSH builder" project that handles all the heavy lifting of building Markdown documentation. It's currently a semi-automatic process that requires some manual setup. However, eventually, the entire process could be automated; I've already [managed to (web)hook into RubyGems](https://hooks.poshtui.com/).

Most of the work has been with the TUI client.

The TUI is able to parse the `Gemfile.lock` file, download all documentation it can find, and present it through a TUI view. While the flow of this entire process is not yet properly defined, the features for each step seem to be working.

Here's a quick rundown of new features:
- `posh docsets` to download all available docsets.
- `posh docsets --clear` will remove all docsets from the system.
- `posh gemfile` to view project gems with the ability to search and pick a docset for review.

Additionally:
- The client now stores a local cache to avoid redownloading docsets multiple times.
- There are many stability improvements to the client itself and better error handling.
- Underlying libraries have been updated; the biggest win here is that the markdown renderer has received numerous improvements. This should generally help to present documentation better.