Firefox: Allow to specify the custom location to Firefox binary

[miry]

Sample code:

capabilities = Selenium::Firefox::Capabilities.new
capabilities.firefox_options.args = [
  "--headless"
]
capabilities.firefox_options.binary = "/Applications/Firefox\ Developer\ Edition.app/Contents/MacOS/firefox"
driver.create_session(capabilities)

References: https://developer.mozilla.org/en-US/docs/Web/WebDriver/Capabilities/firefoxOptions