[Feature request, dotnet-codegencs] Be able to reuse code for templates

Hi!

First of all, thank you very much for the tool! It is easy to use, it's simple and it keeps saving a lot of time for our team.

Second, I need an advice on how to to reuse code for templates while keeping "text template + codegen tool" approach. We do use dotnet-codegencs tool to scaffold boilerplate code for our projects. Something like `dotnet new` but with some logic involved. It will be nice to be able to reuse basic logic and do not copypaste it across csx templates.

What I have tried so far

## Use `#load` directive
Sadly, this option is not supported. The `TemplateBuilder` supports only `#r` directives.

## Pass multiple templates as args to the dotnet-codegencs tool
Not supported, there currently can be only one template

## reference local dll (not nuget package)
```
#r "ScaffoldHelpers.dll"
...
```
or
```
dotnet-codegencs template run .\TestTemplate.csx -r .\ScaffoldHelpers.dll
```
Fail with
```
Unhandled exception: System.TypeInitializationException: The type initializer for 'TestTemplate' threw an exception.
---> System.IO.FileNotFoundException: Could not load file or assembly 'ScaffoldHelpers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
```
the reason is that referenced assemblies are not copied into the build directory and are not loaded by TemplateLoader.

## Write custom runner using TemplateBuilder / TemplateLoader classes
Sadly, these are not distributed as a nuget packages, only as a part of dotnet-codegencs tool

## Fork the dotnet-codegencs
We are currently using custom fork but it looks like a hack, I'd prefer to keep using original tool:)

Thanks in advance, hope you have a good Holidays!