refactor(scan): custom headers

[psyray]

fix #19

I've refactored all the code that generate headers
Given the case that each tool has his proper format, although it is the same between multiple tools, I've decide to write some functions to manage every case properly.

Also, now 2 formats are compatible :

• old one, string, like custom_header: 'Cookie: Test', for backward compatibility
• new one, a dictionary, more flexible, will replace the string format

custom_header: {
  'Cookie':'Test',
  'User-Agent': 'Mozilla/5.0',
  'Custom-Header': 'My custom header'
}

Tested and working
Waiting feedback and test