all: Switch the --lang syntax to use argv instead

It was a bit awkward using `mgmt run lang --lang <input>` so instead, we
now drop the --lang, and read the positional argv for the input.

This also does the same for the --yaml frontend.

vagrant/motd

@@ -9,6 +9,6 @@ To get started, try:
 
 $ cdmgmt
 $ make clean build
-$ ./mgmt run --tmp-prefix lang --lang examples/lang/hello0.mcl`
+$ ./mgmt run --tmp-prefix lang examples/lang/hello0.mcl`
 
 Enjoy!