Now use pure globbing to generate 100% valid function and
completion paths, effectively splitting the init process in two
steps, one which paths are added, and other when initialization
is done (sourcing init).

Package initialization code is done in a new function called
`initialize`, which takes care of the dependencies, ensuring
proper `init.fish` call order and handling compatibility.

This initialization code introduces a new interface for
`init.fish` hook, which deprecates the previously used event
model. The new interface injects three variables into `init.fish`:
path, package and bundle. This variables can be used by the
package to autoload paths, use bundled files, etc.

Also supports key bindings by sourcing
$OMF_CONFIG/key_bindings.fish and also key_bindings.fish in
packages (plugins and themes) root directories. This is done
when fish_user_key_bindings is called.

The features added by this function are not used anywhere.

Fix #151

Address the use case of autoload path removal, which allows themes
and packages definitions to be unloaded from running fish session.

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.

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.

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

Swap `prompt_segments` and `refresh` description.

This was actually trying to load PATHs that do not exist under $OMF_CONFIG
directory and sourcing files that did not exist.