Add filterPlaceholder input

[Crevil]

This PR closes #121

Additions

• filterPlaceholder input to allow setting the filter input placeholder
• Sample on demo application
• Feature to vNEXT in changelog
• Input to README component input list

Changes

• It seems the line endings have been changed on select-dropdown.component.ts, I've only added line 37 (@Input() placeholder: string)
• Changed tabs to spaces for OPTIONS_MANY

[basvandenberg]

Thanks for the contribution, I guess this only applies to the single select case, right?

Since there are quit a few changes I'll have a look at it later this week.

[Crevil]

Yeah, gotta admit that I havent checked out the multi selection mode, so it properbly is.

Sure thing, glad to help out and thanks for the project. If you'd like it, I'd gladly help out with further developments, just let me know.

[basvandenberg]

Thanks again @Crevil! Great work, even added an example, very nice. Your pull request has been merged into the beta.4 branch.