v2: Add parsing strictness configuration

[mmcdole]

Overview

Add configuration options to control parsing strictness, allowing users to choose between robust/permissive parsing (default) and strict standards-compliant parsing.

Tasks

Define Strictness Options

• Identify specific behaviors to make configurable:
  • Date format parsing (allow non-standard formats vs RFC-compliant only)
  • Required field validation (ignore vs error on missing required fields)
  • Unescaped markup handling (attempt to parse vs error)
  • Unknown elements (ignore vs error)
  • Namespace validation
  • Character encoding issues

Implementation

• Add flags to StrictnessOptions struct
• Update parsers to check strictness settings
• Ensure default remains permissive for backward compatibility
• Document each strictness option clearly

Example Usage

opts := &gofeed.ParseOptions{
    StrictnessOptions: gofeed.StrictnessOptions{
        AllowInvalidDates: false,     // Reject non-standard dates
        AllowMissingRequired: false,  // Error on missing required fields
        AllowUnescapedMarkup: false,  // Don't try to fix broken markup
    },
}
feed, err := parser.Parse(reader, opts)

Benefits

• Users can choose appropriate level of strictness for their use case
• Helps identify feed quality issues when needed
• Maintains backward compatibility with permissive defaults
• Better standards compliance when required

Related Issues

• Part of v2 RFC (RFC: gofeed v2 – Proposed Changes #241)
• Works with error handling (v2: Improve error handling with typed errors and context #255)