Simplify `omf cd` code

This also namespace the function to `omf.packages`.

refact(pkg/thefuck): use a better, oh-my-fishy thefuck plugin

update some cli completions: remove query and version. add doctor

Rework omf theme code: Integrate with fish_config, ditch refresh calls

- Move theme installation to cli entry point (omf.fish)
- Generalize XDG_CONFIG_HOME code in `omf.xdg.config_home`
- Generalize prompt conflicts check in `omf.checks.fish_prompt`
  * Migrate `omf.doctor` to use it

+ Change theme activation steps
  - Create user function path if not existent
  - Check for conflicts in fish_prompt, aborts if found
  - Remove current theme from autoloading paths
  - Add target theme to autoloading paths
  - Link target theme's fish_prompt to user's
  - Persist selected theme to $OMF_CONFIG/theme

This approach brings two major advantages:

- Theme prompt now shows accordingly in fish_config web interface
- Faster theme changes, reducing it from seconds to some milis
- Avoids calling refresh, which makes users much more happy

Return 0 when package installs, != 0 on error

bundle.add: fix theme being persisted twice

`omf theme` was persisting the installed theme twice, one time on
`omf.install` and other time on `omf.bundle.add`.

Bring plugin back to oh my fish organization

Convert plugin to new OMF architecture

Fix broken documentation link

Fix #101

install: Remove existing fish_prompt by backuping

Works toward generalizing the backup restoration logic using a
function, then restoring both config.fish and fish_prompt.

First works toward generalizing file backup code, applying to both
config.fish file and now fish_prompt.fish too. This avoids clean
installs which start with theme issues.

Implement `autoload -e` option to remove from autoload paths

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

Forgot to suppress error message

Fishy packages

Instead of having every plugin function inside the same file, which
increases the loading time unnecessarily and is not fishy, source the
`uninstall.fish` file for the plugin before the `uninstall_$plugin_name`
event is emitted.

Update zh-CN readme header

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