Check files inside node_modules

[neslinesli93]

Hi, thank you for your work on elm-analyse :)

This PR aims to solve #236, which afaik is an intended behaviour which solves #22, an old issue you opened up yourself. This is a proposal to rollback that behaviour, for two reasons:

• the NoUnusedDependencies check cannot be configured nor disabled in any way at this moment
• the fact to ignore node_modules can be easily configured from elm-analyse.json, and I think nowadays is more common to have a couple folder with elm sources and that can be manually added to elm-analyse.json rather than using . as the scope for all elm files

Finally, I've also added a Dockerfile so that the project can be built easily, in spite of all the versions of node/npm/typescript one may have on their own system.

EDIT: the windows build fails because it fails to install elm-platform. Any clue?