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

package.yaml

@@ -47,6 +47,7 @@ library:
   - sqlite-simple
   exposed-modules:
   - CAStore.Command
+  - CAStore.Command.Autocomplete
   - CAStore.Command.Type
   - CAStore.Config
   - CAStore.Config.CLI