Read search paths from configuration in didChangeConfiguration, update on the fly

[jakebailey]

With #1289, we can now do what I mentioned in #794:

• Don't read searchPaths in initialization options (note: need to preserve backward compat for a time).
• Accept our own list (or lists) of paths in the main configuration sent in didChangeConfiguration.
• Read PYTHONPATH from the environment.
• Combine info ourselves.

This would mean:

• We have one fewer thing "required" in our initialization (Defaults should really be defaults #469).
• We can update the search paths and reload on a configuration change (PLS doesn't handle changes to python.autoComplete.extraPaths setting #507).

But, before doing this, we'll need to settle on our choices of configuration options, as python.autoComplete.extraPaths is likely not what we'd want to call it.