Support stdin for Configuration Input

[jtdub]

Problem

Configuration files must be provided as file paths. This prevents piping configurations from other tools or using here-docs.

Proposed Solution

Support "-" as a special filename to read from stdin:

napalm get_config | hier-config-cli remediation --platform ios --running-config - --generated-config intended.conf

Limitation: Only one of --running-config or --generated-config can be "-" since stdin can only be read once.

Acceptance Criteria

• "-" reads from stdin for --running-config
• "-" reads from stdin for --generated-config
• Error if both are set to "-"
• Works with pipes and here-docs
• Documentation updated
• Tests added