Add a function to automatically decompress GZIP resources

[federkasten]

This PR adds a function to automatically decompress GZIP resources.
You can specify the decompression behavior using :pack key in cavia's profile.

Also, this PR contains a patch that fixes an integer overflow issue around handing content-length.

[totakke]

Thank you for good PR. The unpacking feature is very convenient.

But :pack key name is misleading. Someone may understand it that downloaded resources will be packed.

I think that :unpack or :packed key name is better. Please fix the key name only.

[federkasten]

Thank you for the review.

I have modified a key for automatic decompression, from :pack to :packed.

[totakke]

Thank you. I've just merged this PR.