Add zsh completion definitions for Metasploit utilities

This PR adds zsh completion definitions for the msfconsole, msfencode and msfvenom utilities. The list of encoders is included in them because it does not change as frequently as other module types. These have all been tested with zsh version 5.0.6. Steps to install additional zsh completion definitions can be found in the [zsh-users/zsh-completions repository](https://github.com/zsh-users/zsh-completions#manual-installation).