Flymake indicator

1. [Enhancement] Show error/warning/info for checkers.
2. mouse-1 click to show flycheck menu.
3. Show vcs segment in both active and inactive windows.
4. Show checker indicator in both active and inactive windows.
5. Optimize performance for flymake segment.
6. Fix: flymake mouse-clicks.
7. Improve some help-echo and mouse-clicks.
8. [Feature] Flymake indicator.
9. Fix #82: load async if it exists but is not loaded.
10. Other minor improvements.