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/format/mod.rs

@@ -4,6 +4,8 @@ use ::util::interrupt;
 
 pub mod stream;
 
+pub mod chapter;
+
 pub mod context;
 pub use self::context::Context;