Add a two character shortcut ('jk' by default) to exit insert-mode

Merges the relevant parts of https://github.com/marijnh/CodeMirror/pull/2719 to add the key combo 'jk' to exit insert mode. You can set the keys you want to use by editing lines 645-6 of vim.js, e.g. to use 'ii':

    firstEscCharacter: 'i',
    secondEscCharacter: 'i',

If using a repeated key (like 'ii' above), you may also want to change the escSequenceTimeout to say 200ms on line 649 - I personally found it difficult to reliably press the same key twice that fast :)

    escSequenceTimeout: 200,

N.b. macro recording using this shortcut won't work, so just use ESC to exit insert-mode when recording macros.

Additionally to disable this functionality set line 641 to false

    enableEscKeymap: false,

Here is the now closed issue on CodeMirror for more info about the original patch. https://github.com/marijnh/CodeMirror/issues/1990

SO link with (possibly) slightly more info: http://stackoverflow.com/a/24997067/104264