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.

docs/faq.md

@@ -212,7 +212,7 @@ requires a number of seconds as an argument.
 #### Example:
 
 ```
-./mgmt run lang --lang examples/lang/hello0.mcl --converged-timeout=5
+./mgmt run lang examples/lang/hello0.mcl --converged-timeout=5
 ```
 
 ### On startup `mgmt` hangs after: `etcd: server: starting...`.