Add new `is numeric` test

[lindseydiloreto]

For whatever reason, fabpot doesn't think it's worth adding an is numeric test to Twig. I happen to strongly disagree.

twigphp/Twig#1861

Would you mind adding a test like this to Twig Perversion?

{% if myVar is numeric %}
    {# Do something which requires a number #}
{% endif %}

[marionnewlevant]

added in 1.0.1

[lindseydiloreto]

Brilliant!! Thanks Marion! :)