Add shortcut for enabling Firefox' headless mode

[franzliedke]

This provides a dedicated API for enabling headless mode. This is already possible via adding command-line arguments.

See #4591.

Please comment on whether using headless! conforms with your conventions. 😄
Do we need a way to disable this feature as well? Since there is no remove_argument, I would not assume so...

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[lmtierney]

LGTM, @p0deje ?

[titusfortner]

I'm against this idea. I don't think we should add a special method for each browser argument.

[franzliedke]

@titusfortner Apparently, this was requested and not shot down in #4591.

[p0deje]

Java bindings also provide this, so even I agree with @titusfortner that we should not wrap every argument with custom method, I'm more concerned to make bindings API consistent to each other.

Thank you for contribution! Merged in a2a7634.

[barancev]

headless is special :)

[jimevans]

I’m not entirely sure I agree that “headless is special.” There is already a mechanism for passing arguments to a browser’s command line. There is no defensible reason to not include a type-safe switch for every command line argument of a browser, if an exception is made for one of them. I am perfectly happy with inconsistency between languages if that means the particular language binding’s API retains internal consistency.