Add label to new taggable item in dropdown

[Turbotailz]

This adds a label beside new options in the drop-down list to help distinguish it from other options. Example:

The text can be changed by setting the prop new-option-label to any string. It can be disabled by setting the string to "".

[coveralls]

Coverage remained the same at 96.429% when pulling c22caee4e5b92477ed18aee43621bd832ca3e806 on Turbotailz:taggable-label into 8ef15a12d395878105959b1c80456e4f1bbe1701 on sagalbot:master.

[msonowal]

@sagalbot This is a must have when push tag is used, We also update the UI to have plus sign for example at before. So imo, I think at-least having a new css class to be applied as in this PR for the tags that is created or can be created so we can play with the CSS.

[MohamedAliTera]

Any chance this gets merged?