Change all whoami calls for $USER variable

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.
Change all pwd calls for $PWD variable

This avoids spawning additional processes as $PWD **always** contains
the current working directory.
Change all hostname calls for $HOST or $SHORT_HOST
Merge pull request #3223 from mcornella/use-special-vars

Use special variables where we can
Add CONTRIBUTING.md file general structure
Categories of issues in oh-my-zsh (work-in-progress)
Format categories of issues in oh-my-zsh (still WIP)

Conflicts:
	CONTRIBUTING.md