Add option for always showing gzip diff

[jtadmor]

Hi! Awesome library. One thing that would help us - while we have a budget set, seeing the raw size of the gzip diff is almost always wanted, whereas the current setup hides it if the budget is set.

I just added an option for allowing this.

[hashicorp-cla]

All committers have signed the CLA.

[lourd]

This looks awesome @jtadmor! @brkalow, could you review this please? My team would also really appreciate this feature. I believe it would also resolve #6 that you filed?

[jzxhuang]

@brkalow @dstaley at a glance it looks like this would work still, any chance it could be merged? showing the raw size diff would be much appreciated, it's definitely more intuitive to me than the % difference!