Refactor command execution and help

- Copy and improve help text, and convert it to AsciiDoc files
- Evaluate commands dynamically

Usage:
  omf search (-p/--plugin | -t/--theme) <name>  Search for a plugin or theme

Examples:
  omf search -p nvm
  omf search -t bobthefish
  omf search vi

Be more careful about restoring backups (fix #509)

following the 'Also in' pattern (PT-BR)

Limit search to name+description

Add US flag to Brazilian Portuguese README

Detect themes by fish_prompt instead of name

* Add SHA256 checksum for installer

* Fix Travis

Handle missing theme setting more gracefully

Get oh-my-fish installer from a secure connection.

Use HTTPS schema instead of cleartext HTTP.
SSL certificate provided by letsencrypt.com
Installed on private nginx server for redirection.

Update release date

Replace package database with independently installed package repositories

- Add support for fetching and installing from package repositories containing package URLs and metadata
- Support multiple package repository sources
- Add new repo command
- Remove submit command
- Enhance describe and search commands
- Fix and improve help text for new and adjusted commands
- Fix bundle install exit status
- Refer to plugins as "plugins" instead of the general term "packages"
- Code clarity improvements
- Put plugin/theme distinction into package metadata

Docs on how to set variables in `init.fish` (#328)

Fix #328

* Ignore '.git' suffix on theme URLs.
* Regenerated the Themes doc (after a long hiatus)

* Update authors list

* Add release notes for v4

* add ukrainian translation

* fix UA translate in readme

* fix UA translation in faq

* fix UA translation in packages.md

* fix documentation link