withResponsiveImagesIf() on conversions

[rcerljenko]

Can we also have newly added withResponsiveImagesIf() method on conversions? Eg:

$this->addMediaConversion('thumb')->withResponsiveImagesIf(/* some condition */);

[freekmurze]

If the implementation is simple, I'd accept a PR with tests and an updated readme. I'll only merge if this can be solved in an easy way.

[rcerljenko]

here you go: #2086