Move away from fish_greeting for initialization

[sagebind]

Using fish_greeting to define functions and run initialization code for themes should be considered unreliable and breaks in the following cases:

• User switches themes during the middle of a terminal session. The shell is not reloaded, and so fish_greeting is not executed.
• User has a custom fish_greeting variable or function, overriding the theme's default greeting. Again, the theme's fish_greeting is not executed.

The recommended approach is to lazy-load defaults using set -q ...; or set -g ... and to place functions in individual files in a functions directory.

See the discussion in oh-my-fish/oh-my-fish#483.