Provide API to open a new folder (fixes #58)

* Provide API to open a new folder (fixes #58)

* extract more generic command

* use underscore to prevent command from showing in keybindings list

src/vs/workbench/api/node/extHostApiCommands.ts

@@ -162,6 +162,13 @@ class ExtHostApiCommands {
 				{ name: 'column', description: '(optional) Column in which to preview.' },
 			]
 		});
+
+		this._register('vscode.openFolder', (uri: URI) => this._commands.executeCommand('_workbench.ipc', 'vscode:windowOpen', [[uri.fsPath]]), {
+			description: 'Open a folder in the current window. Note that this will shutdown the current extension host process and start a new one on the given folder.',
+			args: [
+				{ name: 'uri', description: 'Uri of the folder to open.', constraint: URI }
+			]
+		});
 	}
 
 	// --- command impl