Merge branch 'master' of github.com:ng-select/ng-select

fix: ngClass overwrites 'ng-select' class (ng-select#1543)

closes ng-select#1535

fix(style): fixed color in base styles (ng-select#1491)

closes ng-select#1454

fix: emit empty string for default term length (ng-select#1478)

closes ng-select#1420

feat: group selection no longer affects disabled items (ng-select#1434)…

… (ng-select#1437)

Merge branch 'master' of github.com:ng-select/ng-select

fix(appearance): apply styles correctly

closes ng-select#1422

feat: add appearance property for material theme (ng-select#1413)

closes ng-select#1334

Merge pull request ng-select#1410 from ng-select/improvements

Improvements

feat: expose DropdownPosition type (ng-select#1400)