Provide a way to optimize all resource attributes in a tag #466

The following use-case outlines the use of multiple resource attributes in a 
single '<img>' tag.
The attributes are 'src' and 'data-loadsrc'.  Today, mod_pagespeed seems to 
grab the first one and doesn't have a way to understand that it should pay 
attention to the second resource.  

THe ModPagespeedUrlValuedAttribute does give you the kind of configurations you 
would need to understand that so that you wouldn't have to pay the cost of 
looking ahead once you've found the first resource attributre. E.g. 
'ModPagespeedUrlValuedAttribute img data-loadsrc Image' 

<img src="foo.jpg" title="Foo" width="950" height="300" 
class="slide-on-viewport" 
data-loadsrc="/cms_assets/images/photos/promo/foo1.jpg"/> <div 
class="foo"></div>

<img 
src="data:image/gif;base64,R0lGODlhAQABAJH/AP///wAAAMDAwAAAACH5BAEAAAIALAAAAAABA
AEAAAICVAEAOw==" title="Foo" width="950" height="300" class="slide-on-viewport" 
data-loadsrc="/cms_assets/images/photos/promo/foo1.jpg"/> <div 
class="foo"></div>