Report total length of base64-encoded data

[macbre]

• report base64Length
• add to offenders base64 data longer than 4 kB (encoded)

[macbre]

• Data URI Performance: Don't Blame it on Base64
• When to base64 encode images (and when not to)

[macbre]

Will emit offenders for base64-encode data bigger than 4 kB:

[
      ".wikia-bar-seasons { background-image: ... } // base64: 5.27 kB, raw: 3.95 kB",
      ".WikiaSeasonsPencilUnit { background-image: ... } // base64: 104.06 kB, raw: 78.04 kB",
      ".WikiaHeader.WikiaSeasonsGlobalHeader .wikia-header-mask { background-image: ... } // base64: 6.01 kB, raw: 4.50 kB"
]