Please add --licensedir flag to cabal-install

Currently, cabal install license files into the location given by `--docdir`. Many distributions, however, have a special location into which license files (and nothing else) should be placed, i.e. both SUSE and Fedora use `/usr/share/licenses/PKGNAME` for that purpose. It would be nice if we could have Cabal install those license files into the correct location right away, instead of having to move the files around after the fact.