add base64 for osx

pages/osx/base64.md

@@ -0,0 +1,19 @@
+# base64
+
+> Encode and decode using Base64 representation.
+
+- Encode a file:
+
+`base64 -i {{plain_file}}`
+
+- Decode a file:
+
+`base64 -D -i {{base64_file}}`
+
+- Encode from stdin:
+
+`echo {{plain_text}} | base64`
+
+- Decode from stdin:
+
+`echo {{base64_text}} | base64 -D`