Isolated template components

[notramo]

While the include template tag is useful for some scenarios, a different approach is needed for actual applications.
I recommend adding a component tag.
Differences from include:

• Components can not access variables in the global template scope, only variables provided by context producers.
• Components have to specify which parameters they need (and whether a given parameter is required or optional with a default value).
• Instantiating a component without a required parameter would throw error.
• Components can have slots.

[treagod]

I like the idea.

What would be interesting is how and where to define the component and how to include the component. Using a path to the component file seems odd to me. Possible solution could be to have a component directory from where Marten loads the components in a registry with a name as their key.

Then you could load the component by refering to its name.

I created #165 for discussion