Add the article 'parts' subdivision, as marked on the TODO list

[NaelsonDouglas]

This PR solves the following problem which is cited on the TODO list

Many WikiHow articles also contain "Parts" which break down further into sub-steps. Write a function to parse these additional divisions.

Now the Step object has a field 'part' which keeps the name of the "parent topic" of that object

Example:
Let's say we have an article on this format

Part 1
Step1=Do A
Step2=Do B

Part 2
Step3=Do C
Step4=Do D

Then we can use the package normally, but when accessing 'step1' we can check where it lives using step1.part, which in this case is 'Part 1'. The same result would fetched when calling 'step2.part'

[JarbasAl]

thanks for sending a PR!

at first sight this looks good, I will only be able to test later but if everything checks out i will get it merged and make a new release