Show message when user click on extract button but no url have been provided.

[sophiekovalevsky]

@prabhakar267 Currently, if you do this, the default message appears and we can improve it.
If you want I can take this and do it through jQuery, if that it is okey for you, or if you have any other approach would be awesome to put the ideas here.

[prabhakar267]

@sophiekovalevsky I think we can work on this now since #31 has been merged 😄

[sophiekovalevsky]

@prabhakar267 Indeed.