Add confirmation to API.

Server---Build-level-API.md

@@ -16,6 +16,9 @@ package sbt
 sealed trait UIContext {
   /** Prompts the user for input, optionally with a mask for characters. */
   def readLine(prompt: String, mask: Boolean): Option[String]
+  /** Ask the user to confirm something (yes or no) before continuing. */
+  def confirm(msg: String): Boolean
+
   // TODO - Ask for input with autocomplete?
 
   /** Sends an event out to all registered event listeners. */