Adding support for content-specified template files

Basically we should have a way to specify custom templates on the content file, such as:

~~~
Title: My Custom Page
-----
template: my_custom_template
~~~

The template file (on the `template` variable) will be loaded if:

1. The `intendedTemplate` is `false`.
2. If the template file, defined in the `template` attribute is available and exists.

This is very handy for special cases, for example, when we have a catalog of products and we want to have one special template file for a single product (not all of them).