WIP: Resolve "Fix Marketo Extractor"

Closes #104 (closed)

• write a utility that generates a config file containing credentials and start_date (each resource can be run independently)
• write a client.py that can deal with authentication/sending requests (MarketoClient)
• add a method for getting activity types
• add a method for getting leads
• add a method for getting activities
• write a utility that generates the marketo keyfile
• write a utility that generates the initial state file (based on the job's start time)
• implement mypy in the makefile
• get all code passing mypy checks
• add a method that dumps out the schema and data using singer core (marketo utils, not client)
• write a README
• write some tests
• create a dockerfile
• write a makefile with useful commands

Future improvements? (for another MR/umbrella issue)

• also supports an end_date where it will check responses and stop loading data at a certain time
• concurrency
• utilize more generators so the whole response doesn't need to be stored in memory (scalable)
• ability to choose which fields to load per endpoint