Typo: Update example to use correct command

When creating new plugins the word 'pkg' does not exist for 'omf new' command.
The correct one is 'omf new plugin'.

Update changelog for v6

Fix #529

Readme updates

Notes for fish 2.2 kept in a new subsection.

Fixed License shield link in README.md

bump LICENSE year

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.