Improve documentation of the target attribute of step elements.

[kemaldaggen]

As discussed #105, current documentation makes people think that target parameter only accepts id of element or DOM element itself. In fact this is not true. This pull request contains extended documentation of behavior of target parameter.

[kevzettler]

lgtm

[timlindvall]

Let's merge this in for sake of maintenance, though we probably should just remove this folder and use GH Pages as the source of truth moving forward. (I'll open a separate issue for this.)

[brondsem]

It would be nice if this could be published to http://linkedin.github.io/hopscotch/ I was about to make a similar pull request to update the target docs, but then noticed this was done already, just not published.

[timlindvall]

Good call. Per #288, I'd like to kill off the /demo folder in master, so we have one source of truth for documentation. Part of that process should be making sure the content between the two sources agree with each other.