Added SSVE support for nested conditionals

New PR for #1164

.NET RegEx engine supports matching nested patterns.

More information: <http://retkomma.wordpress.com/2007/10/30/nested-regular-expressions-explained/>

```html
<html>
    <head></head>
    <body>
        @If.SayHello;
            Hey 
            @If.ShowName; 
                @Model.Name 
                @If.Zalgo;
                    HE COMES
                @EndIf;
            !
            @EndIf; 
            Bye!
        @EndIf
    </body>
</html>
```

Should output:
```html
<html>
    <head></head>
    <body>Hey Bob! Bye!</body>
</html>
```