Add namespace loader

[alexander-schranz]

• bug fix / new feature? [Feature Request] Support for multiple paths #314
• BC break? no
• doc PR: nette/docs#???

This introduce a namespace loader. This allows to use a namespace like app:: similar to Plates and Blade template engine.

Usage:

    $defaultLoader = new FileLoader(__DIR__ . '/templates');
    $emailLoader = new FileLoader(__DIR__ . '/emails');
    $someLoader = new FileLoader(__DIR__ . '/vendor/some/some/templates');
    $otherLoader = new StringLoader(['main' => 'othercontent']);

    $loader = new NamespaceLoader([
        '' => $defaultLoader,
        'email' => $emailLoader,
        'some' => $someLoader,
        'other' => $otherLoader,
    ]);

This way it is very flexible as we can use any loader again.

[alexander-schranz]

@dg looks like you rebased it. Do I need to change the target branch?

Edit: Oh my fault looks like the master was force pushed to another state. I will rebase my branch.