TUI: Use rage binary in example commands if installed

Cargo.lock

@@ -64,6 +64,7 @@ dependencies = [
  "rand 0.7.3",
  "secrecy",
  "sha2",
+ "which",
  "x509",
  "x509-parser",
  "yubikey-piv",
@@ -353,6 +354,12 @@ dependencies = [
  "signature",
 ]
 
+[[package]]
+name = "either"
+version = "1.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e78d4f1cc4ae33bbfc157ed5d5a5ef3bc29227303d595861deb238fcec4e9457"
+
 [[package]]
 name = "elliptic-curve"
 version = "0.8.5"
@@ -1212,6 +1219,16 @@ version = "0.10.0+wasi-snapshot-preview1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1a143597ca7c7793eff794def352d41792a93c481eb1042423ff7ff72ba2c31f"
 
+[[package]]
+name = "which"
+version = "4.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b55551e42cbdf2ce2bedd2203d0cc08dba002c27510f86dab6d0ce304cba3dfe"
+dependencies = [
+ "either",
+ "libc",
+]
+
 [[package]]
 name = "winapi"
 version = "0.3.9"