Add repack example

E.g. this can be used to fix CRC checksums in files obtained
by fuzzing methods.
So if you do this:

RUSTFLAGS="--cfg fuzzing" cargo run --example repack <A> <B>

it will read from A and repack it to B while ignoring any CRC
errors.

Note that the repack example is atm not expected to reproduce
100% the same file: page boundaries are not being respected
so they might turn out differently in the generated file.
This might then in turn mess up the packet <-> absgp link.

But to fix all of this we'd have to add a "end of page" flag
to packets.

examples/repack.rs

@@ -0,0 +1,64 @@
+// Ogg decoder and encoder written in Rust
+//
+// Copyright (c) 2018 est31 <MTest31@outlook.com>
+// and contributors. All rights reserved.
+// Licensed under MIT license, or Apache 2 license,
+// at your option. Please see the LICENSE file
+// attached to this source distribution for details.
+
+extern crate ogg;
+
+use std::env;
+use ogg::{PacketReader, PacketWriter};
+use ogg::writing::PacketWriteEndInfo;
+use std::fs::File;
+
+fn main() {
+	match run() {
+		Ok(_) =>(),
+		Err(err) => println!("Error: {}", err),
+	}
+}
+
+macro_rules! btry {
+	($e:expr) => {
+		match $e {
+			Ok(v) => v,
+			Err(e) => {
+				println!("Encountered Error: {:?}", e);
+				break;
+			},
+		}
+	};
+}
+
+fn run() -> Result<(), std::io::Error> {
+	let input_path = env::args().nth(1).expect("No arg for input path found. Please specify a file to open.");
+	let output_path = env::args().nth(2).expect("No arg for output path found. Please specify a file to save to.");
+	println!("Opening file: {}", input_path);
+	println!("Writing to: {}", output_path);
+	let mut f_i = try!(File::open(input_path));
+	let mut f_o = try!(File::create(output_path));
+	let mut pck_rdr = PacketReader::new(&mut f_i);
+	let mut pck_wtr = PacketWriter::new(&mut f_o);
+
+	loop {
+		let r = btry!(pck_rdr.read_packet());
+		match r {
+			Some(pck) => {
+				let inf = if pck.last_packet {
+					PacketWriteEndInfo::NormalPacket
+				} else {
+					PacketWriteEndInfo::EndStream
+				};
+				btry!(pck_wtr.write_packet(pck.data.into_boxed_slice(),
+					pck.stream_serial,
+					inf,
+					pck.absgp_page));
+			},
+			// End of stream
+			None => break,
+		}
+	}
+	Ok(())
+}