Add virtualenv prompt for robbyrussell theme

[petarnikolovski]

Description

Added virtualenv prompt for robbyrussell theme (same as in the purity theme)

Motivation and Context

Usually when one cd into directory with active virtualenv name of the virtualenv is displayed on the left hand side in parentheses.

How Has This Been Tested?

cd into and out of the directory with virtualenv has desired effect

Screenshots (if appropriate):

https://imgur.com/D0ZG5Nc

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] If my change requires a change to the documentation, I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• [ ] I have added tests to cover my changes, and all the new and existing tests pass.

[petarnikolovski]

@davidpfarrell @gaelicWizard any update on this, is it ready to be merged? :relaxed: