Allow ProgressBarBuilder.setETAFunction to be used externally

ProgressState was package-private and setETAFunction's BiFunction
includes ProgressState, preventing setETAFunction from being used
outside of the package.

Author: deejgregor

src/main/java/me/tongfei/progressbar/ProgressState.java

@@ -8,7 +8,7 @@
  * @author Tongfei Chen
  * @since 0.5.0
  */
-class ProgressState {
+public class ProgressState {
 
     String taskName;
     String extraMessage = "";