add shell completions

[FWDekker]

in fish, when I write git and press tab, fish tells me what options i have. when i write mommy git and press tab, fish doesn't know what's going on!

mommy should add and install integrations for various shells:

• bash
• fish
• zsh

[FWDekker]

completions for fish have been added in f56a368, but no deployment has been added yet so far, because the installation directory differs hugely per system and i still need to figure that out

(note from future: they have been figured out and have been deployed)

[FWDekker]

i took a brief look at bash completions, and honestly they look like so much work to do. they are super verbose to program, and might almost require more complexity than mommy itself. i might implement very simple completions in the future, but if anyone reading is interested in writing bash completions for mommy, feel free to open a pr with the completions (no need to worry about integrating with the build/testing framework, i can do that if needed)