Fix #1016: PowerShell extension doesn't load due to extension conflict

This change fixes an issue that was illuminated by the AzureRM Tools
extension where the `fs.appendFile` method provided by Node.js was not
being called with the correct number of parameters.

The AzureRM Tools extension loads up the AppInsights SDK for Node.js
which in turn loads a module called Zone.js.  Zone.js replaces built-in
Node.js modules like `fs` to instrument their function calls.  The
instrumented version of that module was not as permissive on our missing
`appendFile` parameter so it threw an exception which ultimately
prevented the PowerShell extension from loading.

This issue was fixed by populating the `callback` parameter of the
`appendFile` function so that the instrumented wrapper operates
correctly.

src/logging.ts

@@ -142,7 +142,14 @@ export class Logger {
 
         this.logChannel.appendLine(timestampedMessage);
         if (this.logFilePath) {
-            fs.appendFile(this.logFilePath, timestampedMessage + os.EOL);
+            fs.appendFile(
+                this.logFilePath,
+                timestampedMessage + os.EOL,
+                err => {
+                    if (err) {
+                        console.log(`Error writing to vscode-powershell log file: ${err}`)
+                    }
+                });
         }
     }
 }