Spike: Make source plugin independent of sample app about gatsby-starter

[JanLenoch]

As it turned out, the Git submodules are still highly error-prone, even in projects where a well-known circle of developers collaborate on a regular basis. In projects where broader community is expected to collaborate, it poses too many risks. Forgetting to pull a submodule properly (together with the parent module) might result in overwriting code changes done by someone else.

I've filed a suggestion to Gatsby to have local plugins outside of the apps filesystem structure: gatsbyjs/gatsby#7518 .

Until this is possible, only NPM plugins can be used. I'll split the repo into two (plugin, starter site) and make an NPM package out of the plugin.