Install plugins/ scripts into libexec

Assume that all `data-files` entries beginning with `plugins/` are executables (shell scripts and the like) and install them into`$libexec/git-vogue` directory with `+x` set.

Testing
------------

Installing the package now results in three files (two Haskell executables and one shell script) in `$prefix/libexec/git-vogue/`:

````
$ cabal sandbox init
$ cabal install
$ find .cabal-sandbox -maxdepth 3 -type f -perm -111
.cabal-sandbox/libexec/git-vogue/git-vogue-hlint
.cabal-sandbox/libexec/git-vogue/git-vogue-stylish-haskell
.cabal-sandbox/libexec/git-vogue/git-vogue-cabal
.cabal-sandbox/bin/git-vogue
````