introduce the modular architecture

The core of the extension consists of:
 * Commands registry. It holds the all available commands to
   different modes. To add/delete commands uses map/unmap API methods.
 * Key bindings. The dictionary of already binded keys and corresponding
   handlers. To add/remove bindings uses bind/unbind API methods.

To globals exposed only Vimflowy base prototype and the vimflowy extension
itself. Please use the Vimflowy.prototype or its alias Vimflowy.fn to extend it.
remove trailing spaces
get ESC key back
add skeleton for key bindings
fetch current version from manifest file
remove functions.js

now bind keys at keybindings.js
bump new version
add fetchFileContent function to vimflowy prototype
add help page

All mappings populated at runtime
add styles to help page inspired by vimium help