Adds merging adjacent selectors if body is the same.

Adds merging adjacent selectors if body is the same.

* Skips if selector is vendor specific, i.e. matches /-(moz|ms|o|webkit)-/.
* Will not merge selectors if one selector may not be supported, e.g. :nth-child, :last-child, etc. - configurable via `--selectors-merge-mode/selectorsMergeMode` option.
* Defaults to merging all selectors so `ie8` mode needs to be forced via mentioned options.