Specify an additional library search directory

[moleike]

Add a -J / --jpath <dir> option in the cmdline executable to specify additional library search directories, right-most wins.

The additional paths are tried if it fails to load a module relative to the file it's imported from. Here is how's it's done in the C++ implementation: https://github.com/google/jsonnet/blob/master/core/libjsonnet.cpp#L232