[help] Restructure command list and matching function.
[help] Define a common event handler function for fsm.
[help] Refactor CommandHelper.
[help] Take out html displaying feature as class HelpViewer.
[index] Remove debug displays.
Merge branch 'fsm-refactor' into develop
[help] Move call to helpViewer directly under CommandHelper

Also, privatize some members of CommandHelper and HelpViewer.
Now VimFSM is really an fsm. It does not participate in displaying.
[keys] Create KeysViewer class and its init().
[keys] Implement context-aware key list.
Merge branch 'help-lookahead' into develop