Added hs create vue-app that utilizes vue-cli init command

[miketalley]

This adds the hs create vue-app <destinationFolder> command which generates a boilerplate Vue module app using the vue-cli and the cms-vue-module-template repo. It is the equivalent of running vue init HubSpot/cms-vue-module-template <destinationFolder>.

Fixes #217

[gcorne]

What if vue is not installed?

[miketalley]

Thanks! I had mean't to handle this -- I will look into it.

[gcorne]

What if dest is not a directory?

[gcorne]

What is the purpose of this change?

[miketalley]

So fs.ensureDir in commands/create.js will check if the directory exists and, if not, create it. This was the one case that relied on the folder being created by that. So when I changed it from fs.ensureDir to fs.exists this needed to be added to create the folder.

[gcorne]

One thing that I think is unclear here is that hs is running vue under the hood. I am not 100% sure that this adds a ton of value since the user has to have the vue CLI installed. If it does add value, I think we need to make it as clear as we can that the vue CLI is be run by the hs because if there is a problem it needs to be clear that the problem is with vue not hs.

[miketalley]

I did some more research into vue including the vue init command and the upcoming changes to the vue-cli and vue with the upcoming release of vue v3 which is in RC. It really changed my outlook on using a template to build a vue module project.

With the release of vue v3, the vue-cli will be essentially deprecating the vue init command by moving it into a separate package @vue/cli-init which I think could cause some confusion with the functionality with this PR. We would need to handle checking for these deps(and versions of the vue-cli) to make sure the spawned process that executes the vue command under the hood works properly.

To @gcorne's comment about clarity of using vue under the hood, I think that if this process fails it could be unclear to the user where the problem occurred.

Creating a project through the cms-vue-boilerplate and the cms-vue-module-template both allow subsequent addition of plugins through the vue add command. So if the boilerplate does not contain a specific plugin, it can be added later.

With these points in mind, I'm thinking that the previous iteration of the hs create vue-app command that generates a project using the cms-vue-boilerplate would be a better choice because it is more simple, less complex and more transparent, less magical.

I'm going to close this out in favor of the previous iteration that uses cms-vue-boilerplate which was reverted until we could look into templates more.
New PR: #286