Add user middleware execution order control

[robhrt7]

Since user could have multiple middleware plugins in one SourceJS instance, we should provide control over their execution order.

It could be done with custom configuration, containing names and order number + separation of middleware types.

First time to implement: renderers of custom templates like jade, markdown and etc. Since other middlewares relies on HTML as an input, we should provide some guaranties that they will get it, instead of jade sources.