Add optional argument isDeep to File.clone

[c0]

• Allows passing in isDeep:false to perform a shallow clone.

[coveralls]

Coverage remained the same when pulling db7e066 on c0:optional_shallow_clone into 44ef836 on wearefractal:master.

[yocontra]

Also related #16 and #9

[c0]

To put this in context, cloneDeep can have a negative performance impact as seen in amd-optimize (build went from 18s to 90s). It was bypassed there, but other repos may be affected.

I left cloneDeep true by default to maintain the current API, but it is worth to consider making it shallow by default.

[yocontra]

@c0 Wow 15s to clone a file? That seems insane. We might have other problems if that's the case - we may want to look at using a new method of cloning that's closer to the metal

https://github.com/AlexeyKupershtokh/node-v8-clone

[popomore]

I think it's necessary to set this argument. One for performance, the other some customObj don't want to be deep cloned

file.customObj = {};
var clonedFile = file.clone();
clonedFile.customObj === file.customObj; // should be true

[yocontra]

Fixed by #32