Move $OMF_CONFIG variable to $OMF_PATH/init.fish

This PR sets the default value of `OMF_CONFIG` variable in
framework init.fish file. The variable can still be overridden by
the user by setting it on `~/.config/fish/config.fish file`.

This is in preparation to rewriting install script in plain fish,
which will ditch config template and stop replacing `config.fish`
contents in favor of just appending Oh My Fish startup.

Add test-generate-themes-doc.fish

* Run with Travis CI, if commits contain changes to `db/themes`
* When run; fail build if `docs/Themes.md` is no longer up-to-date

Add getopts pkg

Add kill-on-port pkg

Remove some pushd popd, using git --git-dir --work-tree

Add nelsonjchen theme to db

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)