ngx-input-tag feature requests (different tag formats and prevent creating new tags)

[kevinbuhmann]

Describe the solution you'd like

1. ngx-input-tag should allow the tagFormatter to be set via a input (input overrides provider). We have two types of tags on our site and they use different formats.
2. ngx-input-tag should have an option to disable adding tags that are not in the suggested tag array.
3. ngx-input-tag should have an option to prevent backspace from removing a tag.

I would be happy to open a PR.