Add basic autocomplete support

ca-store now takes a --completion argument to output completions
to standard output. These are to be used for completion scripts,
in future commits.

Before the 1.0.0 release we need to either:
1) Work out how to get bash et al to accept these completions
   without mangling them
2) Get optparse-applicative to handle the command parsing entirely
   and forgo the Moore machine approach

app/Main.hs

@@ -1,8 +1,9 @@
 module Main(main) where
 
 import CAStore.Command (runCommand)
+import CAStore.Command.Autocomplete (generateAutocompletions)
 import CAStore.Config (finalConfig)
-import CAStore.Program (Program, runProgram, getCommand)
+import CAStore.Program (Program, runProgram, getCommand, getAutocomplete)
 import CAStore.Program.IO.Text (err)
 import CAStore.Program.Storage (initialise)
 
@@ -12,4 +13,6 @@ main = finalConfig >>= flip runProgram defaultProgram
 defaultProgram :: Program ()
 defaultProgram = do
   initialise
-  getCommand >>= either err runCommand
+  getAutocomplete >>= \case
+    True  -> generateAutocompletions
+    False -> getCommand >>= either err runCommand