* init: rewrite init process

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).

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.

* omf: migrate to new init hook

* omf/templates: migrate to new init and uninstall hooks

* docs: document new init and uninstall hooks interface

* README: update new hook interface spec