Fixes #886 - better multi pseudo class / element merging.

Fixes #886 - better multi pseudo class / element merging.

Why:

* 4.0 introduced a stricter mergeability rules, however in some
  cases merging is still possible but harder to determine;
* tests suggested all multi pseudo class / element rules can be
  merged unless those contain more than one of the following:
  `:after`, `::after`, `:before`, `::before`, `:first-letter`,
  `::first-letter`, `:first-line`, `::first-line`, or `:lang`;
* there's also a new compatibility switch,
  `{ compatibility: { selectors: { multiplePseudoMerging: false } } }`
  which, if set to false, disables merging for all rules with
  multiple pseudo classes / elements.