Add option to disable Imgur upload

This patch solves #20 (closed).

By default the new option enable_imgur_upload is assigned to true, but If the user don't want the checkbox, it can add the option in ~/.config/xfce4/xfce4-screenshooter assigned to false.

If the option is false:

If the option is true (by default):

Just a note: the diff shows that I've added a lot of lines inside lib/screenshoter-dialogs.c, but I've only added an if and indented the code correctly.

Also keep in mind that we need to document this option to the docs!