Added conditional echoes to blade compiler

This pull request adds the new blade syntax:

```php
{{ $value if $condition }}
```

which compiles to:

```php
<?php echo $condition ? $value : ''; ?>
```

I find myself wanting this a lot, as a shorter, more readable syntax for only echoing a variable if some condition is true (only add this error class if the `$errors` array has something in it, for example).