feat(line): impl Styled for Line (#968)

This adds `FromIterator` impls for `Line` and `Text` that allow creating
`Line` and `Text` instances from iterators of `Span` and `Line`
instances, respectively.

```rust
let line = Line::from_iter(vec!["Hello".blue(), " world!".green()]);
let line: Line = iter::once("Hello".blue())
    .chain(iter::once(" world!".green()))
    .collect();
let text = Text::from_iter(vec!["The first line", "The second line"]);
let text: Text = iter::once("The first line")
    .chain(iter::once("The second line"))
    .collect();
```

src/text/line.rs

@@ -495,6 +495,18 @@ impl std::fmt::Display for Line<'_> {
     }
 }
 
+impl<'a> Styled for Line<'a> {
+    type Item = Line<'a>;
+
+    fn style(&self) -> Style {
+        self.style
+    }
+
+    fn set_style<S: Into<Style>>(self, style: S) -> Self::Item {
+        self.style(style)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::iter;
@@ -612,6 +624,16 @@ mod tests {
         assert_eq!(Style::reset(), line.style);
     }
 
+    #[test]
+    fn stylize() {
+        assert_eq!(Line::default().green().style, Color::Green.into());
+        assert_eq!(
+            Line::default().on_green().style,
+            Style::new().bg(Color::Green)
+        );
+        assert_eq!(Line::default().italic().style, Modifier::ITALIC.into());
+    }
+
     #[test]
     fn from_string() {
         let s = String::from("Hello, world!");