MINOR: qpack: implement standalone decoder tool

Implement a standalone binary to be able to easily a hex-string QPACK
stream. The binary must be compiled via the Makefile. Hex-strings are
specified on stdin.
diff --git a/.gitignore b/.gitignore
index d588b07..a277688 100644
--- a/.gitignore
+++ b/.gitignore
@@ -49,6 +49,7 @@
 dev/tcploop/tcploop
 dev/hpack/decode
 dev/hpack/gen-rht
+dev/qpack/decode
 /src/dlmalloc.c
 /tests/test_hashes
 doc/lua-api/_build