Add a center() convenient method to TextView

Avoids having to import HAlign

examples/progress.rs

@@ -60,7 +60,7 @@ fn coffee_break(s: &mut Cursive) {
     s.pop_layer();
     s.add_layer(Dialog::empty()
         .title("Preparation complete")
-        .content(TextView::new("Now, the real deal!"))
+        .content(TextView::new("Now, the real deal!").center())
         .button("Again??", phase_2));
 }
 
@@ -117,6 +117,7 @@ fn final_step(s: &mut Cursive) {
     s.add_layer(Dialog::empty()
         .title("Report")
         .content(TextView::new("Time travel was a success!\n\
-                               We went forward a few seconds!!"))
+                               We went forward a few seconds!!")
+                 .center())
         .button("That's it?", |s| s.quit()));
 }

src/view/text_view.rs

@@ -101,6 +101,12 @@ impl TextView {
         self
     }
 
+    /// Center the text horizontally and vertically inside the view.
+    pub fn center(mut self) -> Self {
+        self.align = Align::center();
+        self
+    }
+
     /// Replace the text in this view.
     pub fn set_content(&mut self, content: &str) {
         let content = strip_last_newline(content);