Refactor the installer to be more maintainable and interactive. The installer is now more robust and safe in its operation, and asks interactive questions for choices that the user must resolve. Setting up confuguration is also now changed to take advantage of Fish 2.3 features and does not mess with user's configuration files without permission.

- Make installer smarter and use uninstaller in destroy
- Better handling and checking for offline installs
- `omf destroy` simply uses `install --uninstall`
- Final warning message is no longer displayed when user intentionally aborts install
- Backups are restored during uninstall
- Update the README to detail the new ways to install OMF

Fixes #377

gives some fresh air to contributing, reflecting more our current style and redaction.

This fixes a rare case when fish's builtin function
__fish_reload_key_bindings is not defined.

* Add virtualfish plugin to the registry

Fix double key bindings load

Fixed spelling error

makes package docs better by refreshing hooks documentation
and also contributing process.

When I upgraded my omf peco stopped to work I had to change the repo to this one that seems the one the is currently maintainable.

* init: load only key bindings from current theme

* theme: reload key bindings on theme change

Refactor autoload: Split the big function into two smaller ones,
doing only option parsing at main function.

The algorithm is also rewritten, now in two steps for both path
inclusion an exclusion functions: 1) use auxiliary lists to store
valid function and completion paths, 2) bulk insert or remove just
once in the variable.

Now also respects path insertion policy, keeping user function
path always in front of other paths, thus allowing precedence of
user functions.