feat(Paragraph): add alignment convenience functions (#866)

Added convenience functions left_aligned(), centered() and
right_aligned() plus unit tests. Updated example code.

Signed-off-by: Eelco Empting <me@eelco.de>

examples/panic.rs

@@ -128,7 +128,7 @@ fn ui(f: &mut Frame, app: &App) {
         .title("Panic Handler Demo")
         .borders(Borders::ALL);
 
-    let p = Paragraph::new(text).block(b).alignment(Alignment::Center);
+    let p = Paragraph::new(text).block(b).centered();
 
     f.render_widget(p, f.size());
 }