config refactor stage 0 - new base class

[liuzhe-lz]

Description

The first version of ConfigBase was too magic that nobody can understand how to use it.
This refactored version has 4 major improvements:

1. The API is much more intuitive
2. Support validating data type
3. Handle base path transparently, so it no longer pollutes __init__()'s args
4. Automatically unpack nested config, so most subclasses won't need to override __init__()

Note that this PR is merging to dev-config branch, not master.

Checklist

• test case
• doc

How to test

Should be tested together with other configure refactor PRs.