feat: create custom srcset

[ericdeansanchez]

The purpose of this PR is to implement custom srcset behavior by supporting:

• custom min and max widths
• custom width tolerance

This PR also seeks to do this without changing the public API.

This PR has been split into three commits:

• validate min, max, and width tolerance
  • to call one validator instead of three
  • test
• implement custom target widths
  • pull target width generation into UrlBuilder.py
  • this is where the validation occurs
  • test
• custom srcset widths and tolerance
  • validation is done here as well (so that create_srcset and target_widths can be used separately)
  • extend the API to support custom srcset generation by specifying:
    • custom min/max widths and
    • width tolerance.

Note: There have been 220 additions and 25 deletions here, however, more than 117 of theses additions are documentation and comments.

EDIT: The fourth commit improves the test cases for validate_min_max_tol

[sherwinski]

This looks great! I just have a few follow up questions.