omf doctor

In order to pro-actively diagnose an issue on someone's computer, a
`omf doctor' command is being added. This command will initially check
errors that could cause a theme to not be loaded, but in the future more
checks are going to be added to prevent issues from being created for
problems that are fixable by the Oh My Fish user alone.

Remove reload notice from installer script

Also fix a whitespace issue.

Save history before replacing shell process

Fixes an issue with history between shell reloads via refresh function call.
As `exec` replaces the current process in a non-gently fashion, the
persistent history could be out of sync with the in-memory history.

Calling `history --save` before `exec` should force the shell to persist the
history before replacing the current process.

Only emit uninstall_* event on `omf destroy`

When calling `omf destroy`, the `bundle` file is completely erased
because we are calling `remove_package` on each installed package. We
don't want to erase that file, we just want to emit the event and later
on remove the code, which is done with `rm -rf "$OMF_PATH"`.

Organize commands and remove undocumented aliases

install: Replace source by fish invocation

Update installation script instructions

Migrate install script from bash to fish

- Checks for a supported fish version (greater than 2.1)
- Migrate travis install script to fish

Dockerize

Travis does not provide the original repository branch name on any
environment variable. We are getting the information from the GitHub
API instead.

The misleading environment variable `TRAVIS_BRANCH` returns `master`
instead:
> TRAVIS_BRANCH: For builds not triggered by a pull request this is the
name of the branch currently being built; whereas for builds triggered
by a pull request this is the name of the branch targeted by the pull
request (in many cases this will be master).

Reference: http://docs.travis-ci.com/user/environment-variables/

- Install fish from dockerfile and generate a docker image.
- Install Oh My Fish! from downloaded git repository and generate a
  docker image.

Instead of overwriting functions that use `exec fish` on test
environment, skip this call if the variable `CI` has been defined on the
running shell.

This Dockerfile creates an ubuntu machine with fish-shell and the
dependencies we use on our test suite installed.

It does not install Oh My Fish, which one might find weird (I do). Not
confident that this is the best approach, but it's the best I could find
at the moment.

Update pkg urls and add weather plugin

Faster loading

In order to support dependency between plugins a function called
`require` has been added.

Unnecessarily sourcing a `$plugin_name.fish` is affecting the loading
time.

Stash does not support --git-dir

Stash is a shell script that does not support `--git-dir`, not even on
latest versions. We need to use `git -C` which was only added on recent
versions of `git`.

`omf update` will fail in case omf need needs to run `git stash` and
git is not compatible.

Split custom "init.fish" into clearer "before.init.fish" and "init.fish"

add pastfish theme

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.