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.

src/lib.rs

@@ -31,6 +31,8 @@ pub mod format;
 pub use format::format::Format;
 #[cfg(feature = "format")]
 pub use format::stream::{Stream, StreamMut};
+#[cfg(feature = "format")]
+pub use format::chapter::{Chapter, ChapterMut};
 
 #[cfg(feature = "codec")]
 pub mod codec;