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

.gitignore

@@ -6,6 +6,7 @@
 !package.yaml
 !app/Main.hs
 !src/CAStore/Command.hs
+!src/CAStore/Command/Autocomplete.hs
 !src/CAStore/Command/Type.hs
 !src/CAStore/Config.hs
 !src/CAStore/Config/Type.hs