Allow to install plugins from a custom location (url or file location). For example, to install the output of a gradle release run, one can use (assuming extracted under build/distributions):

 build/distributions/elasticsearch-0.15.0-SNAPSHOT/bin/plugin -url build/distributions/plugins/ -install lang-groovy

The -url parameter accepts either a file based location, or a url. For file based location, the structure supported is either a flat dir with the plugin(s) there, or a plugin-name/plugin-file structure.

Note, this is a breaking change since now one can only list a single plugin name in -install, though it can be repeated several times to install several plugins in a single command.