Signify plugin language via `cpp` or `py` parameter

[knoepfel]

Currently, Phlex plugins are loaded via the syntax:

modules: {
  a: {
    plugin: "<plugin path>"
  },
  b: {
    plugin: "<plugin path>"
  },
  ⋮
}

with the assumption that the loaded module is a C++ module. To make loading C++ and Python modules symmetric, the recommended schema is instead:

modules: {
  a: {
    cpp: "<shared object-file path>"
  },
  b: {
    py: "<python module>"
  },
  ⋮
}

Eventually, this should be replaced with a system that can calculate whether the plugin is C++ or Python.