Custom theme boilerplate type

[jamesm131]

It would be great to have a boilerplate option that would create the scaffold for a project with a custom theme. That is including the theme folder in the .vuepress directory and a few example files.

I personally think that this would make it easier to start creating a theme by providing a simple entry point and a well defined structure.

Though I don't have much experience in writing command line utilities or file generators, but would be happy to set aside some time to work on it if this feature is something that you think would be in line with the goals of the project.