Profile the Dhall Command (#608)

This benchmark allows one to run:

```
$ stack bench --profile --library-profiling dhall:dhall-command <<< "/path/to/my-dhall-fille.dhall"
```

And the profiling information will then be output in dhall-command.prof.

benchmark/dhall-command/Main.hs

@@ -0,0 +1,20 @@
+
+{-# LANGUAGE OverloadedStrings #-}
+
+module Main where
+
+import qualified Dhall.Main as Main
+import           Dhall.Binary (defaultProtocolVersion)
+
+options :: Main.Options
+options = Main.Options
+  { Main.mode = Main.Default False
+  , Main.explain = False
+  , Main.plain = False
+  , Main.ascii = False
+  , Main.protocolVersion = defaultProtocolVersion
+  }
+
+main :: IO ()
+main = do
+  Main.command options

dhall.cabal

@@ -321,3 +321,13 @@ Benchmark deep-nested-large-record
         criterion                 >= 1.1      && < 1.6,
         dhall
     Default-Language: Haskell2010
+
+
+Benchmark dhall-command
+    Type: exitcode-stdio-1.0
+    Main-Is: benchmark/dhall-command/Main.hs
+    Build-Depends:
+        base                      >= 4        && < 5  ,
+        dhall
+    Default-Language: Haskell2010
+    ghc-options: -rtsopts -O2