[Feature] Add flag to not clear model on backspace

[Deafsilver]

Is your feature request related to a problem? Please describe.
I have a need to be able to clear the value by clicking the "x". However, when clearable == true and a value is already selected, if the user were to search for a new value and not find it, holding down backspace will erase all characters in the ng-select and clear the value. Having a method to only clear by clicking the "x" would be helpful.

Describe the solution you'd like
Add a flag that if present would prevent the handling of backspace.