create-aragon-app could work on directories with content

[eternauta1337]

🚀 Feature

Right now, create-aragon-app will fail if the target directory already exists. This requirement is reasonable, but it tends to be a major nuisance, for example for directories that have previously been initialized as git repositories. Also, a user may start a project and then decide to make it an Aragon app.

It shouldn't be too hard to allow the bootstrapping in a directory that already exists and contains files and folders, with a few softer requirements like npm not been initialized in the directory, but allowing for a .git, a contracts or other non-blocking structures to exist.