Add Configuration File Support for Default Options

[jtdub]

Problem

Users who repeatedly use the same platform or output format must specify these options every time.

Proposed Solution

Support a configuration file for default options:

Config File Locations (in priority order)

1. --config command line option
2. .hier-config-cli.yaml in current directory
3. ~/.config/hier-config-cli/config.yaml
4. ~/.hier-config-cli.yaml

Example Config

defaults:
  platform: ios
  format: json

Acceptance Criteria

• Config file loading implemented
• Priority order respected
• CLI options override config file
• config show command to display active config
• config init command to create template
• Documentation updated