Issue 6665 - Abstract element hiding container logic into its own module

Based on https://codereview.adblockplus.org/29783618/

This patch separates out common functionality between ElemHide and ElemHideEmulation into a separate "ContentFilterModule" object (which will also work for Snippets eventually).

It does not yet make ElemHideEmulation take advantage of the optimizations in ElemHide, that will be a follow-up patch.

[Manish Jethani, 2018-05-17 05:51:55]

Patch Set 1
Patch Set 2: Rebase

Based on https://codereview.adblockplus.org/29783618/

https://codereview.adblockplus.org/29784555/diff/29784568/lib/elemHide.js
File lib/elemHide.js (right):

https://codereview.adblockplus.org/29784555/diff/29784568/lib/elemHide.js#new...
lib/elemHide.js:214: getSelectorsForDomain(domain, specificOnly = false)
Code that is specific to ElemHide is still in the ElemHide object only.

https://codereview.adblockplus.org/29784555/diff/29784568/lib/elemHideEmulati...
File lib/elemHideEmulation.js (right):

https://codereview.adblockplus.org/29784555/diff/29784568/lib/elemHideEmulati...
lib/elemHideEmulation.js:31: exports.ElemHideEmulation = Object.assign(new
ElemHideTemplate(), {
ElemHideEmulation is now just like ElemHide, but with its own copy of the data
structures containing its own filters. Exceptions are still shared via the new
ElemHideExceptions module.

https://codereview.adblockplus.org/29784555/diff/29784568/lib/elemHideEmulati...
lib/elemHideEmulation.js:40: for (let filter of this._knownFilters)
Note: I am not yet taking advantage of the optimizations in ElemHide for filter
matching. That will require some more refactoring in ElemHideTemplate, it will
be a follow-up patch.

[Manish Jethani, 2018-08-15 21:02:53]

Patch Set 3: Rebase
Patch Set 4: Move base class into lib/contentFilterModule.js

Note that the Snippets module is still on its own for now. We need to make a few
more changes before we can make it extend from ContentFilterModule, which will
be a separate patch.

[hub, 2018-08-23 16:05:39]

https://codereview.adblockplus.org/29784555/diff/29856580/lib/contentFilterMo...
File lib/contentFilterModule.js (right):

https://codereview.adblockplus.org/29784555/diff/29856580/lib/contentFilterMo...
lib/contentFilterModule.js:134: this._knownFilters])
I'd be for using forEach() here.