New port: www/geckodriver

This program provides the HTTP API described by the
WebDriver protocol to communicate with Gecko browsers, such as Firefox.
It translates calls into the Marionette automation protocol by acting
as a proxy between the local- and remote ends.

WWW: https://github.com/mozilla/geckodriver

PR: 216696
Submitted by: Daniel Ylitalo <daniel@blodan.se>
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D9524