Travis-CI: install Env::PS1, required for testing examples/EnvPS1.PS1
Chrome: MAJOR REFACTORING !!!

Huge refactoring for simplification:
- AngelPS1::Chrome::Flags disappears. Flag constants (Reset, Bold...)
  now return AngelPS1::Chrome objects.
- AngelPS1::Chrome::Color objects have now the same layout as
  AngelPS1::Chrome. Inheritance now applies also to the internal
  representation.

This refactoring will also allow to implement ${} overloading.
Chrome: add ${} overloading

Implement ${} overloading. This allows to use Chrome expression easily
in strings:
    say "normal ${ Red+Bold } RED ${ +Reset } normal";
Chrome: simplify operator overloading decl in Chrome::Color
31-chrome.t: more tests (notes actually)