Optimizer specific config interface

[tkmnet]

Is your feature request related to a problem? Please describe.
It is related to #133. Some cases want to define optimizer-specific configs like is contained in the PR.
In the current implementation, config.py is modified when defining the specific configs in a custom optimizer. It is inconvenient.

Describe the solution you'd like
I want an interface for the specific configs in config.yml that do not need modifying config.py.

One solution is to allow optimizers responsible values in the optimize group of config. In this case, the default values are managed in an optimizer.
But, the conflict avoidance method is maybe wanted like the following implementation.
E.g.

• Only allowing a dictionary that named to same as the optimizer name.

optimize:
  search_algorithm: "foo.barOptimizer"
  "foo.barOptimizer":
    param0: 0.0

print(config.specific.get("param0", default=0.0))