Support binding letter sequence to ESC in OtherModes

[pciazynski]

• Click thumbs-up 👍 on this issue if you want it!
• Click confused 😕 on this issue if not having it makes VSCodeVim unusable.

The VSCodeVim team prioritizes issues based on reaction count.

---

Is this a BUG REPORT or FEATURE REQUEST? (choose one):

Environment:

• VSCode Version: 1.13.1
• VsCodeVim Version: 0.9.0
• OS: Ubuntu 16.04

What happened:
I try to bind jk to <ESC>:

"vim.otherModesKeyBindingsNonRecursive": [       
  {
    "before": [ "j", "k" ],
    "after": [ "<Esc>" ]
  }
]

but this doesn't work. jk does nothing in Visual Mode.

What did you expect to happen:
Because I have binding jk to leave Insert Mode I would like also have possibility to escape Visual Mode with jk, but this doesn't work.

How to reproduce it:
Paste above snipper into your settings.json, go to Visual Mode and press jk. Unfortunately nothing happen, we will stay in Visual Mode.