Support add-ons

[edouard-lopez]

Related: inspired by #355

Current pure.fish version goes a bit behind the original Zsh implementation and it bothers me. Moreover, I want to keep pure simple to maintain and integrating new section for every tool/context/OS is behind the scope of the project itself.

However, I and other feel the need for such features and we should provide a mechanism to support custom section integration so pure can be extended.

Again the goal is not to offer a full blown solution like Tide items or others but a KISS one, and encourage people to take part in the community and its ecosystem.

Approaches

• call a generic _pure_user_addons function during prompt rendering that user can edit
• call a list of functions provided by the user similar to what @jorgebucaran proposed in hydro
• other ideas ?