Minor bug fixes and features

[sethfaulkner]

My team supports a private project and has run hundreds of json files through your project. It works very well for our purposes, with some minor fixes and features added. What is addressed in this pull request is:

• Fix to min_diff method, preventing it from finding actual minimum
• Added None check in transform method, preventing crash
• Added a 'percent_diff' rule to allow the project to match on slightly different values (some of our outputs are not always exact)
• Added the float comparison logic to the '_list_content_diff' method to be able apply rules to them

One thing to note is that I believe there is a slight problem with the list compare logic. It will check length and that each element exists but would be defeated by the case [1,2,2] == [1,1,2].

Thanks for all the hard work on this! I hope you consider this PR.

[rugleb]

Hi! Thx for contribution.
Can u split u PR for a few different PRs? 1 feature/fix = 1 PR?