drive-by fix for minify options brokeness (replacement for PR#69)

[iamjochem]

this is a replacement PR for #69 ... (also see #66 for reasoning).

Little Rant: I (& a million others) have spent man-months of wasted time debugging problems related to shitty handling of options/config objects. where "shitty" is defined as applying one or more of the following:

1. Not cloning [option] objects before passing them to a lib/function.
2. Not cloning [option] objects when they are passed into your/a lib/function (and then changing the given reference!?!?)
3. Some module/lib being really helpful and throwing exceptions because "option property 'foo' is invalid/unknown"

1. & 2. are easy to fix - Object.assign() is your friend - if everyone would just do this we'd be free of so many bugs related to objects passed in as parameters being changed by the callee.

3. - this one requires a change of mindset so that we start treating options object parameters as collections from which known/valid option properties are extracted; this is in constrast to the heavy-handed use of, for instance, JSON schema to validate a given options object and throw an esoteric Error if the options are "invalid" ... which is really helpful when your using a lib indirectly (e.g. babel from the context of uglifyify)

[hershmire]

Any update on when we can get a fix merged in?

[mwiencek]

I upgraded uglifyify from 3.0.1 to 4.0.3 and it stopped minifying my code completely (no visible errors). Applying this patch fixes it. Can it be merged?