Add an option to show debug fragment borders

This is quite a bit cleaner than abusing the rust debug functionality.
If we start collecting too many debugging options in the servo
executable we could opt to organize them into a single option.

Fixes #2263.
auto merge of #3746 : mrobinson/servo/debug-fragment-borders, r=pcwalton

This is quite a bit cleaner than abusing the rust debug functionality.
If we start collecting too many debugging options in the servo
executable we could opt to organize them into a single option.

Fixes #2263.