Add where command

Hi,

This add the _where_ command that print the path to the libraries given in arguments.
Proposition 11 from https://github.com/HaxeFoundation/haxelib/issues/146,
would be useful when you write tools that need the path of a library without having to parse the output of the `haxelib path` command.

For the help message I copied the one from _path_,
but I think that the message from _path_ should be the one to change since it doesn't print libraries paths but their (and their dependencies) classpath/ndll path/-D.
Not sure what to replace the message with though.