Add projectile-last-known-project variable

[bbatsov]

Summary

• Adds projectile-last-known-project variable that tracks the root of the most recently accessed project
• Set automatically by projectile-acquire-root, which is called by most Projectile commands
• Allows users to reference the last project from non-project buffers (e.g., *scratch*, *Messages*)

Closes #1809

Test plan

• Visit a file in a project, run any Projectile command — projectile-last-known-project should be set to that project's root
• Switch to a non-project buffer — projectile-last-known-project still holds the previous project root
• Switch to a different project — variable updates to the new project root