Add support for assembly based (single) templates

[pauldeen79]

Currently, we support the following types:

• Single template, parsed from text and then compiled into a single class in an assembly which can be executed
• Code generation assembly, with multiple code generation providers that are all run and put into a MultipleContentBuilder

We want to add this type:

• Single template, referred to using an assembly name and class name

This type can be a simple template, or internally use a MultipleContentBuilder and return that. The differences are:

• The template is already compiled into an assembly, and doesn't need to be parsed (which gives you far better performance)
• You can include multiple resources, assembly references and so on. These don't have to be resolved at run-time

We have to think of a command to run this template. Or maybe just think of a name for this kind of template.

What about t4plus run -a project\bin\debug\my.dll -t MyNamespace.MyClass Arg1:Value1 Arg2:Value2
-a | --assembly
-t | --type
-u | --use directory (optional)

In the run command, either the filename or the assembly and typename are required.

Advantages:

• Single file, which is useful for example for global templates. Just store the dll (and possible nuget package reference dlls) in one directory, no need for a tree structure with includes or child templates
• Better performance, no need to parse and compile
• Can also be packed into a nuget package (maybe we can add support for this later... t4plus install https://nuget.org/mytemplate --version 1.0.0

You can use these assembly templates for both running and extracting parameters.