Install 'subcommands' into libexec

Switch to a custom build-type, with a `Setup.hs` which overrides the copying of files during the install process. This places the "main" (i.e. first) executable in `$bindir` (i.e. the usual place), but moves the secondary executables into `$libexecdir/git-vogue`.

````
$ cabal sandbox delete
$ cabal sandbox init
$ cabal install
$ find .cabal-sandbox/ -maxdepth 3 -type f -perm -111
.cabal-sandbox/libexec/git-vogue/git-vogue-cabal
.cabal-sandbox/bin/git-vogue
````