config(fish): add fish completions for cli tools

dot_config/fish/functions/mdcp.fish

@@ -0,0 +1,8 @@
+function mdcp -a url
+    if test -z "$url"
+        echo "usage: mdcp <url>" >&2
+        return 1
+    end
+  curl.md $url | wl-copy
+  wl-paste | mdfried
+end