[5.1] Escape triple bracket echos

[eklundchristopher]

I came across a scenario using vue.js where I needed to return unescaped HTML from a data property (http://vuejs.org/guide/index.html#Mustache_Bindings), and to do so, I need to use the triple bracket syntax ({{{ var }}}), but since Blade also uses that syntax, it was an obvious clash.

Seeing as how the double bracket syntax ({{ $var }}) allows you to escape it by prefixing it with an @ symbol, I thought we might as well allow it for the triple syntax for whenever the need arises. (Unless there is a specific reason for it not being allowed already?)

So, in short, this PR allows for the following syntaxes:
{{{ $var }}} outputs <?php echo e($var); ?>
@{{{ $var }}} outputs {{{ $var }}}

[plashenkov]

It looks like this PR does not cut the "@" sign which is used to escape brackets. I.e. @ will go to the output.