[#17] wrap all the command handlers in an object bound to each editor

Rather than trying to find the active editor from the event context
which no longer works in the same way with the react editor, wrap all
the handlers in a class that is initialized with a reference to the
editorView and the editor.
Merge pull request #24 from demmer/separate-handlers-per-editor

[#17] wrap all the command handlers in an object bound to each editor