Add option to start in insert mode

[shawn-stripe]

In order to initialize an editor in insert mode, I don't see any way other than simulating keypress events like so:

const vim = initVimMode(editor, statusBar);

const browserEvent = new Event('keydown');
browserEvent.keyCode = 39;
browserEvent.key = "i";

const wrapperEvent = new Event('keydown');
wrapperEvent.keyCode = 39;
wrapperEvent.browserEvent = browserEvent;

vim.handleKeyDown(wrapperEvent);

Could this be made into a higher-level API like a flag passed to initVimMode? I'd also be happy with instead offering an API to interact directly with the internal vim state, which would be powerful.