added webdriver_manager support to auto-install gecko webdriver

[yanncabral]

Thus, there is no need to manually download and install the Gecko Webdriver compatible with the user platform.

[kkristof200]

Hey, first of all thanks for the pr. Does this lib download/install/use a new geckodriver even if I have one installed already?
Also a better aproach would be:

• get a geckodriver_path(Optional[string] = None) on the init
• check if the variable is not None, and if there actually something located at that path
• if not, check if there is geckodriver installed to the default_path (if default_path != geckodriver_path) (/usr/local/bin/geckodriver on mac)
• if still nothing, use auto-innstall

This approach leaves the ability to the user to have control over what version of geckodriver they would like to use