Imlement 'meta' extension statement

[flexferrum]

Implement 'meta' statement which allows to define inside template some metainformation, accessible from the outside template. For instance, template:

{% meta json %}
{
    "output-type": "xhtml",
    "generator": "some-generator"
}
{% endmeta %}

after parsing:

Template tpl;
tpl.Load(...);
auto& meta = tpl.GetMetaData();
auto generatorType = meta["generator"]; // "some-generator"
auto outputType = meta["output-type"]; // "xhtml"