[FEATURE] Identifier as additional wrapper class name

This introduces an additional configuration option `identifier`
which will be used as a class name of the outermost wrapper div.
Changing the currently available `className` option is destructive
because most CSS stylings affect the default value of `"floating-tip"`.
This new `identifier` option allows additional stylings for special
purposes.
Merge pull request #27 from wishbase/PR/AdditionalClassName

[FEATURE] Identifier as additional wrapper class name