[Pre-Alpha] Pronto runner for flake8

Pronto runner for flake8, a Python Style Guide Enforcer. What is Pronto?

Configuration of pronto-flake8

• flake8 should be in your path or virtual environment.
• pronto-flake8 can be configured by placing a .pronto_flake8.yml inside the directory where pronto is run.

Following options are available:

Option	Meaning	Default
flake8_executable	flake8 executable to call.	flake8 (calls flake8 in PATH)

Example configuration to call custom flake8 executable:

# .pronto_flake8.yml
flake8_executable: '/my/custom/path/flake8'