Implement plugin system

[jsfehler]

This solution uses Pluggy https://pluggy.readthedocs.io/en/latest/ to add a plugin system into Splinter.

This PR features hooks for:

• registering external drivers. This implements #164 and provides a solution for #25 and #730. Internet Explorer is an old browser at this point, but a custom driver could be dropped in without hacking splinter's internals.
• Performing actions after a page is navigated to
• Performing actions before closing the browser.

Plugins classes or modules can be registered via a call to splinter.plugins.register()

This PR is missing:

• Hooks implementation for non-webdriver drivers
• Documentation

But will be added if the overall approach works.

[jsfehler]

This is a bare bones example of what an external driver plugin could look like:

import splinter
from splinter.driver.webdriver import BaseWebDriver


class WebDriver(BaseWebDriver):
    driver_name = "Ie"

    def __init__(self, wait_time=2):
        self.driver = Ie()

        super(WebDriver, self).__init__(wait_time)


class InternetExplorerPlugin:
    @splinter.hookimpl
    def splinter_prepare_drivers(drivers):
        drivers['internet_explorer'] = WebDriver


splinter.plugins.register(InternetExplorerPlugin())