feat: add base64 format #4

This commit adds the option to output base64. Note that this converts
numbers to base64, not strings. formatting 65 will output "QQ==", the
same as using 'echo -ne "A" | base64' would (A is 65 in ASCII).

@@ -16,4 +16,5 @@ keywords = ["cli"]
 anyhow = "1.0.83"
 clap = { version = "4.5.4", features = ["derive"] }
 clap-num = "1.1.1"
+fast32 = "1.0.2"

@@ -41,2 +54,4 @@
     }
 }
+
+fn u128_to_u8_slice(mut num: u128) -> Vec<u8> {