Allow passing a path to --userscripts

[Manishearth]

r? @kmcallister

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4418

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[metajack]

Assigning @kmcallister since @Manishearth pinged him.

[kmcallister]

I am the one who asked for this feature :)

[kmcallister]

Putting all this logic in HTMLHeadElement seems bad. Let's change that to call into a separate userscript module.

[kmcallister]

A mis-typed path is silently ignored? I think it'd be better to panic, or at least print a warning message.

[kmcallister]

r=me with those changes. Thanks for doing this, it'll help with #5260.

[Manishearth]

r=kmcallister

[bors-servo]

saw approval from kmcallister
at Manishearth@f41acb5

[bors-servo]

merging Manishearth/servo/userscript_path = f41acb5 into auto

[bors-servo]

Manishearth/servo/userscript_path = f41acb5 merged ok, testing candidate = afbc51a

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/1248
success: http://build.servo.org/builders/linux2/builds/1246
success: http://build.servo.org/builders/android/builds/1282
success: http://build.servo.org/builders/mac1/builds/1461
success: http://build.servo.org/builders/mac2/builds/1464
success: http://build.servo.org/builders/gonk/builds/427

[bors-servo]

fast-forwarding master to auto = afbc51a