Travis: Base of travis test

Add tmux-zen plugin to db

README: add custom config file documentation

Remove src attribute from spacer image in the README

Moving and renaming install_fish.sh to travis_install_fish

Moving travis_* from tests to tools.

rename: fish_user_function/ -> functions_override/

renaming travis_test.fish to test_runner.fish

Cache from 7.51MB to 1.63MB

Adding apt dependencies + build script + cache directive.

Addresses #40 by briefly documenting init.fish file.

Spacer still works just as good.

This will make Chrome and its siblings not display a broken image placeholder
in the spacer.

Confirmed OK on Firefox and Safari.

DMCA Agreement

This is the message sent to GitHub with for the reconciliation:

> I have talked to @bpinto and his other collaborators and decided that is
> the best for everyone to reconcile and conclude our litigation amicably
> with the following conditions:
>
> * The license read Copyright (c) 2015 Oh My Fish!
> * AUTHORS file.
> * Bruno and Justin remain as admins.
> * Scorphus, derekstavis and bucaran become collaborators.
> * We swear not to fight over copyright issues again.
>
> You may have the DMCA revoked at your earliest convenience.

Fixes on omf destroy and omf.remove_package

 using XDG_CONFIG_HOME

destroy was broken because of a premature refresh. (giving false hang
behaviour)

Replace gitter with slack

We are using slackin (https://github.com/rauchg/slackin) to automatically accept people's invitation.

Travis integration for slack

Command: `travis encrypt "..." --add notifications.slack`

registry: add nvm plugin

Add aggregate theme documentation and a script to generate them

* Generates `docs/Themes.md` that can be used
  for preview available themes

README: Include update instructions

registry: add plugin fzf

registry: add ssh-term-helper plugin

Add grc plugin

Swap `prompt_segments` and `refresh` description.