Add `jsdoc_insert_unknown_return_type` setting

Currently, DocBlockr will insert a return tag if it hasn't "ruled out" the possibility of a function return. If it hasn't determined a return type, but hasn't ruled out a return, then it adds a `@return` tag with no type.

This isn't always desirable behavior, as it is often easier to add a new `@return` tag on the fly, then to have to do a complicated delete that doesn't break the formatting (you either have to do a shift-home delete backspace backspace backspace, or select with a mouse, neither of which are fast).

I added a setting (defaulted to not change the current behavior) that will change whether or not a `@return` tag is inserted if no return type could be determined.

| jsdoc_insert_unknown_return_type | Return Confirmed | Return Unknown | Return Ruled Out | 
| ------------- | ---------------------------------- | --------------------------------- | --------------------------------- |
| true (default - old behaviour) | add tag | add tag | don't add tag |
| false | add tag | don't add tag | don't add tag |