Show slatepack QR codes (#655)

* Show slatepack QR codes

* Make slatepack QR codes optional

Co-authored-by: cliik <cliik@example.com>

Cargo.lock

@@ -1544,6 +1544,7 @@ dependencies = [
  "lazy_static",
  "log",
  "prettytable-rs",
+ "qr_code",
  "rand 0.7.3",
  "remove_dir_all 0.7.0",
  "ring",
@@ -2866,6 +2867,12 @@ dependencies = [
  "unicode-xid 0.2.2",
 ]
 
+[[package]]
+name = "qr_code"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5520fbcd7da152a449261c5a533a1c7fad044e9e8aa9528cfec3f464786c7926"
+
 [[package]]
 name = "quick-error"
 version = "1.2.3"