Feature - Browser: Add experimental configuration and stub out UX

[bryphe]

This change uses the new layer API to create a BrowserLayer, which renders an electron webview over a buffer. This allows us to spin up browser windows, but using the same window management we're accustomed to from Vim.

To enable this functionality, you'll need to set experimental.browser.enabled to true. You can test it out by clicking one of our URLs like the onivim homepage in Help. In addition, you can run this in console: Oni.commands.executeCommand("browser.openUrl", "https://www.onivim.io") to navigate to a custom page. You can do some cool stuff with this - like register a command or bind a key to open localhost for previewing, etc.

Here's what things look like right now:

Lots of things remaining...

• Buttons are not functional - want to hook up a redux store
• Address bar not functional
• Loading spinner doesn't work
• No keyboard control currently
• Tab bar should reflect name/title