[Feature Request] Extend region scanning functionality (CSS selector)

[git70]

Hi!

Your page scanner is currently the best of all in AMO!

I only miss two features present in SiteDelta for example:

1. Ability to add more than one CSS Selector in the "Scan a part of the page" section.
   For example, the "Add Next" button which creates a new CSS Selector field.

2. Ability to set "Do NOT scan part of the page" (in SiteDelta it is called Exclude Region).
   Operation and appearance exactly the same as in point 1 above, but inverse.
   Useful when I need to scan the whole page but excluding a specific area (because it changes too often and is irrelevant)

Cheers!