chore(typeahead): add support typeaheadMinLength = "0" in Typeahead

[hongbo-miao]

There is an API of Typeahead in ng2-bootstrap.

typeaheadMinLength (?number=1) - minimal no of characters that needs to be entered before typeahead kicks-in. Must be greater than or equal to 1.

Right now, it should be greater than or equal to 1. Please support 0. So when the input gets focus, it will show automatically.

Like angular-ui supports this. And one demo using 0 works well.

typeahead-min-length $ (Default: 1) - Minimal no of characters that needs to be entered before typeahead kicks-in. Must be greater than or equal to 0.

Thanks

[tzielins]

A useful feature to present the default options.

[valorkin]

good feature, will be done