[Feature Request] add an option to specify templates' location

[badersur]

The scaffolding tool is great but it's tied to use bootstrap 3 and tweaking generated files for every project is annoying and time consuming! It would be nice to be able to pass a template's folder and the engine looks for templates inside it and use whatever available and use the current default template's path as a last resort!

I would love to provide the path and have the engine to look for templates as follows:

1. use the provided path to look for the full template name (i.e. 2.0.0/Templates/ViewGenerator/... as used inside ~/.nuget)
2. look for the file name only (for convenience and it's easier to create)
3. read appsettings.*.json to know the template's folder (if I didn't provide a path)
4. read some configuration file about custom template's folder (eg. ~/.aspcoreconfig?)
5. use the default location

I'm thinking about a similar approach for dotnet new to provide a different _layout.cshtml that uses bootstrap 4 or any other framework!

[bricelam]

One way to do this, that would be a lot like the old way of doing it, would be to modify TemplateFoldersUtilities to include the root project directory in its search path. (Probably requires a few more changes than just that, but you get the idea.)

After that, you would be able to add a Templates\MvcLayout\_Layout.cshtml file to your project that would be used instead.

Admittedly, this isn't very useful for files like _Layout.cshtml that are only generated once since you could just as easily add it to its final location. But it could be useful for things like views and controllers.