Implement snap packaging, snaps are universal linux packages (#170)

This patch implements the necessary details to package gallery-dl as a snap that can be run on a broad range of supporting GNU/Linux distributions[1].

[1] https://snapcraft.io/

Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

.gitignore

@@ -57,3 +57,12 @@ docs/_build/
 
 # PyBuilder
 target/
+
+# Snap packaging specific
+/snap/.snapcraft/
+/parts/
+/stage/
+/prime/
+
+/*.snap
+/*_source.tar.bz2