Adds logic to manage suppressions

[gavin-truework]

• Adds calls to list, add, and delete suppression support
• Adds doc pages

I wanted to open this to add some basic support for managing suppressions in Postmark. This just adds a few methods to manage them based on the provided API.

I am not 100% sure how I should test this. Do you have a testing account setup that the test should attempt to list, add, and remove a suppression to? I also am not super familiar with docs, so please let me know if I need to do something else.

[Z33DD]

@Stranger6667 Check this later