Enhance default iTerm/zsh theme

[nhoizey]

See:

• https://medium.freecodecamp.org/how-you-can-style-your-terminal-like-medium-freecodecamp-or-any-way-you-want-f499234d48bc
• https://medium.com/the-code-review/make-your-terminal-more-colourful-and-productive-with-iterm2-and-zsh-11b91607b98c