Fix shell crash (#297)

* Updated README

Added explanation for missing parameters (background, title and image)

* Implemented changable icons for AppleScriptTouchBarItem

AppleScriptTouchBarItem now allow to specify any number of icons which can be changed from the script. You cannot change icon from touch event. To change icon, you need to return array from your script with 2 values - title and icn name. More info in readme

* Fixed error related to ShellButton

When you execute Process from swift you cannot relay solely on pipe.fileHandleForReading.readDataToEndOfFile()
Sometimes when I close notebook I get exception saying that you cannot access process.terminationStatus variable while process is running.
Apparently it seems that this call can be finished when OS X put disks into sleep mode(?)

What I did:
1. Added Process.waitUntilExit() call
2. Added timeout (equal to the update interval)

Co-authored-by: Fedor Zaitsev <lobster@Fedors-MacBook-Pro.local>

MTMR/ShellScriptTouchBarItem.swift

@@ -62,11 +62,19 @@ class ShellScriptTouchBarItem: CustomButtonTouchBarItem {
         
         let pipe = Pipe()
         task.standardOutput = pipe
+        
+        // kill process if it is over update interval
+        DispatchQueue.main.asyncAfter(deadline: .now() + interval) { [weak task] in
+            task?.terminate()
+        }
+        
         task.launch()
         
         let data = pipe.fileHandleForReading.readDataToEndOfFile()
         var output: String = NSString(data: data, encoding: String.Encoding.utf8.rawValue) as String? ?? ""
         
+        //always wait until task end or you can catch "task still running" error while accessing task.terminationStatus variable
+        task.waitUntilExit()
         if (output == "" && task.terminationStatus != 0) {
             output = "error"
         }