v2: Implement ParseOptions foundation for parser API

[mmcdole]

Overview

This PR implements the foundation for ParseOptions as specified in #244. It establishes the basic structure and API changes needed for v2, with some features left for future implementation.

What's Implemented

Core ParseOptions Structure

• Created ParseOptions struct with all fields from the RFC
• Added RequestOptions sub-struct for HTTP configuration
• Added StrictnessOptions for parsing behavior control

API Updates

• All parsers now accept *ParseOptions parameter (can be nil)
• Changed ParseURL to context-first: ParseURL(ctx, url, opts)
• Removed ParseURLWithContext - context is now always required
• Updated translators to accept ParseOptions for future extensibility

Basic Features

• MaxItems limiting - implemented at parse level (not translation)
• KeepOriginalFeed - stores format-specific feed in Feed.OriginalFeed
• UserAgent and AuthConfig support in ParseURL
• Client and Timeout configuration

Left for Future PRs

• Conditional requests (IfNoneMatch, IfModifiedSince) - see #247
• HTTP response metadata collection - see #247
• ParseDates toggle
• StrictnessOptions implementation

Breaking Changes

1. All Parse methods now require *ParseOptions:

   // Old
   feed, err := parser.Parse(reader)
   
   // New
   feed, err := parser.Parse(reader, nil)  // nil for defaults
   

2. ParseURL is now context-first:

   // Old
   feed, err := parser.ParseURL(url)
   feed, err := parser.ParseURLWithContext(url, ctx)
   
   // New
   feed, err := parser.ParseURL(context.Background(), url, nil)
   

Testing

All existing tests have been updated and are passing. The API is ready for use, though some ParseOptions fields are not yet implemented.

Closes #244