Add option to only generate hashtags if a specific condition is met

> Some things are better left unsaid
Some strings are better left undone
Some hearts are better left unbroken
Some social media is better left unhashtagged

This PR adds the option to create a guard (either a lambda or a method) that's checked before creating hashtags - the example given in the `README` and specs is pretty basic, but in practice this works well for more advanced checks on relationships and permissions and such.