BREAKING CHANGE:

Command line arguments and some config options and chan spec options have been
changed. It may break scripts and integration tests that depends on the
command line interface.

- Command line argument `--config|-c` is now a top option, which should be
  specified before sub-command.
- Remove command line arguments `--data-dir` and `--chain`. They should be
  configured via the config file.
- Add a new config option `data_dir`, which configures the path of data
  directory.
- Option `ckb.chain` now only accepts a path.
- Option `system_cells` in chain spec now only accepts paths.

The path `data_dir` and `ckb.chain` can be an absolute path, or relative to
the directory containing the config file.

The paths of `system_cells` in chain spec can be absolute, or relative to the
directory containing the chain spec file.

If `--config|-c` is omitted, the app tries `ckb.json`, `nodes/default.json` in
order.

It is recommended to use `nodes/default.json` for development, which can be
created by copying `nodes/default.example.json`. The default config saves data
in `nodes/default/`, and loads chains spec `nodes/spec/dev.json`.

Following locations are obsoleted data directories, which can be removed on
development machines:

- Linux
  - `$XDG_DATA_HOME/ckb`
  - `$XDG_DATA_HOME/NervosDev`
  - `$HOME/.local/share/ckb`
  - `$HOME/.local/share/NervosDev`
- macOS
  - `$HOME/Library/Application Support/ckb`
  - `$HOME/Library/Application Support/NervosDev`