fish_greeting in themes override custom changes, due to the order fundle loads themes

Hi,

i have opened the following issue in bobethefish: https://github.com/oh-my-fish/theme-bobthefish/issues/136#issuecomment-375981625

I think, this happens due to the order fundle loads themes and plugins, so that the hardcoded `fish_greeting` in bobthefish overrides my custom `fish_greeting` in my `fish/functions` folder. The functions folder apparently gets evaluated before the `config.fish` loads fundle. Do you have any idea for a workaround?

Start with the loading order described in config.fish and the fish/functions folder, then compare it with the linked bobthefish issue and the fish_greeting function it references. Reproduce how fundle loads themes and plugins, and confirm whether a theme overrides the user's custom function; done means documenting or correcting the loading behavior.

Written by the indexing model from the issue text.