format: add support for Chapters

Read chapters from an input context and add chapters to an output context.

Note: unlike avformat_new_stream, the equivalent function for chapter is private:
avpriv_new_chapter (part of libavformat/internal.h). I couldn't find any other
solution but re-implementing it in format::context::output::add_chapter.

examples/chapters.rs

@@ -0,0 +1,52 @@
+extern crate ffmpeg;
+
+use std::env;
+
+fn main() {
+	ffmpeg::init().unwrap();
+
+	match ffmpeg::format::input(&env::args().nth(1).expect("missing input file name")) {
+		Ok(ictx) => {
+			println!("Nb chapters: {}", ictx.nb_chapters());
+
+			for chapter in ictx.chapters() {
+				println!("chapter id {}:", chapter.id());
+				println!("\ttime_base: {}", chapter.time_base());
+				println!("\tstart: {}", chapter.start());
+				println!("\tend: {}", chapter.end());
+
+				for (k, v) in chapter.metadata().iter() {
+					println!("\t{}: {}", k, v);
+				}
+			}
+
+			let mut octx = ffmpeg::format::output(&"test.mkv".to_owned()).expect(&format!("Couldn't open test file"));
+
+			for chapter in ictx.chapters() {
+				let title = match chapter.metadata().get("title") {
+					Some(title) => String::from(title),
+					None => String::new(),
+				};
+
+				match octx.add_chapter(chapter.id(), chapter.time_base(), chapter.start(), chapter.end(), &title) {
+					Ok(chapter) => println!("Added chapter with id {} to output", chapter.id()),
+					Err(error) => println!("Error adding chapter with id: {} - {}", chapter.id(), error),
+				}
+			}
+
+			println!("\nOuput: nb chapters: {}", octx.nb_chapters());
+			for chapter in octx.chapters() {
+				println!("chapter id {}:", chapter.id());
+				println!("\ttime_base: {}", chapter.time_base());
+				println!("\tstart: {}", chapter.start());
+				println!("\tend: {}", chapter.end());
+				for (k, v) in chapter.metadata().iter() {
+					println!("\t{}: {}", k, v);
+				}
+			}
+		}
+
+		Err(error) =>
+			println!("error: {}", error)
+	}
+}