el-get-describe: Show install path and provide `el-get-cd'.

If the package is installed `el-get-describe' will now show the
install path and provide an interface similar to `el-get-cd'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #1364 from ruediger/el-get-describe-cd

el-get-describe: Show install path and provide `el-get-cd`.