Added method isTemplateString

[mcaskill]

The very simple method allows template loaders to validate if a given value is maybe the contents of a template or just a template identifier (file path).

Prevents the loader from passing what could be a raw template through further procedures meant for a template identifier.

[mducharme]

I'm ok with the implementation but we got to find a better name... :P

isFile
isLoadable
isSourceString
...?

[coveralls]

Coverage increased (+0.07%) to 94.048% when pulling 0395a6e on mcaskill-patch-2 into 42fc85f on master.

[mcaskill]

I was initially calling it isTemplateLoaded() but I thought since this might not be true 100% of the time.

[mducharme]

What about isLoadable or isTemplateFile..?

[mcaskill]

It could be. The methods would then need to be:

AbstractLoader

protected function isTemplateFile($ident)
{
    return strpos($ident, PHP_EOL) === false;
}

MustacheLoader

protected function isTemplateFile($ident)
{
    return strpos($ident, '{{') === false && parent::isTemplateFile($ident);
}