Consolidate the code that saves the config file
Consolidate the code that saves the config file

Remove ConfigSubcommand._write_config() and move the code into
CLIConfig.save_config().  Before this change, we created ~/.brkt only
when calling ConfigSubcommand.  Callsites that called
CLIConfig.save_config() would fail when ~/.brkt didn't exist.

brkt/brkt-cli,brkt/brkt-cli