Add possibility to set a range for the feed

[ursbraem]

If you have various tiles on your site, maybe some news tiles and then some excerpts from your insta feed, you will welcome the possibility to specify a range in your plugin.

I have added that to the flexform, to the show template as well as to the backend preview (which in this commit only shows the chosen number of posts (max 8) and a tad larger)

[ursbraem]

Need to add a condition for the firstPost setting first to avoid fluid error