Sections: look in a Spaceship config directory for section defs

[olets]

Description

Adds support for storing all custom sections in a single directory known to Spaceship. Two primary motivations:

• ergonomics: rather than manually importing section definitions into .zshrc, users can just save files to the directory Spaceship checks and be done with it
• section installation: this change makes it possible for section developers to write installation scripts which add section definitions without messing with the user's .zshrc. This could help cut down on the "add a section for X" PRs, as well as opening the door for more complex or bespoke sections that might not be appropriate for core

Default location follows the ~/.config convention. Puts sections in a sections subdirectory to leave room for anything else someone might want to put in the config directory in the future.

Added README documentation, following the same reasoning as in #762 that talking up the customizability can only make Spaceship sound even better, and that having more documentation will help users who aren't experienced with shell scripting.