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.
Refactors property merging into a separate Optimizer singleton.
Adds property overriding so a more coarse property overrides a more granular one.

* E.g. 'border-top' overrides 'border-top-width' and 'border-top-color'.
* Another example is 'margin' overriding 'margin-(bottom|left|right|top)'.
Adds reducing non-adjacent selectors.

* Removes repeated properties in same selectors which are not next to each other.
* E.g. '.one{color:red;margin:0}.two{color:#fff}.one{margin:1px}' will become '.one{color:red}.two{color:#fff}.one{margin:1px}' because margin is repeated.