Add configurable retry logic for transient failures

[ewimsatt]

Description

This PR implements retry logic for handling transient failures, as requested in #254.

Changes

• Added three new configuration options:

  • maxRetries (default: 2) - Number of retry attempts
  • retryDelay (default: 100ms) - Initial delay between retries
  • retryStatusCodes (default: [408, 429, 500, 502, 503, 504]) - HTTP status codes that trigger retry

• Implemented exponential backoff: delay * (2 ^ attemptNumber)

• Retries on transient HTTP status codes (timeouts, rate limits, server errors)

• Retries on network errors (ECONNRESET, ECONNREFUSED, ETIMEDOUT, etc.)

• Preserves existing HEAD->GET retry behavior

Example Usage

const blc = require('broken-link-checker');

const options = {
  maxRetries: 3,           // Retry up to 3 times
  retryDelay: 200,         // Start with 200ms delay
  retryStatusCodes: [429, 500, 502, 503]  // Customize which codes to retry
};

Benefits

• Reduces false positives from flaky servers
• Handles temporary network issues gracefully
• Configurable to match different use cases
• No breaking changes - defaults maintain current behavior

Fixes #254