Non-screen-clearing update support

New ShowScreenOpts() function, which takes a new ScreenOpts struct, to control
various aspects of the ShowScreen behavior. This should now be used in favor of
the deprecated ShowScreen() and ShowScreenNoResponse() functions, and allows
for the additional of future options without needing to continue making new
functions or breaking the public API.

README.md

@@ -18,7 +18,7 @@ Everything I know about 3270 data streams I learned from [Tommy Sprinkle's tutor
 Usage
 -----
 
-See the example folders for a quick demonstration of using the library. The examples are very similar, but example1 uses the lower-level function ShowScreen(), and example2 uses a higher-level function HandleScreen().
+See the example folders for a quick demonstration of using the library. example1 uses the lower-level function ShowScreen(), and example2 uses a higher-level function HandleScreen(). example3 demonstrates updating the client's 3270 display while waiting for a response by using an update thread and a waiting response thread.
 
 Here's [a video introducing the library][introVideo] as well.