Add [else], [elseif] shortcodes and an ability to use negative conditions

Hi, Super Interactive team!

My name is Maks. I've seen you want to add some features to your plugin and did it myself.

So, currently with your plugin you can use  [else] and [elseif] shortcodes and negative expressions. See example below:


```
[if qs="x:1"]
X is equal 1
[elseif qs="x:2"]
X is equal 2
[elseif qs="x:3"]
X is equal 3
[elseif qs="!x:4"]
X is not equal 4
[else]
Another cases
[/if]
```

You can use any number of elseif conditions. The code needs some testing, I'm going to do this a bit later. 
If you are interesting in further development by me just let me know. I am going to add a lot of conditions, ability to use regexps, ranges and a lot of WordPress conditional tags.