fix: add typeahead css class binding on host

[anjmao]

fixes #81

[coveralls]

Coverage decreased (-0.01%) to 88.228% when pulling 04dcddb on add-more-css-classes into 6a1ede6 on master.

[anjmao]

Example for showing placeholder when dropdown is opened and typeahead is used

ng-select.typeahead.opened .ng-placeholder {
   display: block !important;
}

ng-select.typeahead.opened .ng-value {
   display: none !important;
}

[coveralls]

Coverage decreased (-0.01%) to 88.228% when pulling dbf1ac3 on add-more-css-classes into 6a1ede6 on master.

[coveralls]

Coverage decreased (-0.01%) to 88.228% when pulling 57463a4 on add-more-css-classes into 6a1ede6 on master.