Unify empty line at end of file with editorconfig

Introduce an [editorconfig] configuration file to tell developer editors that we want an empty line at end of file.

After making softwares and component uses a buildout.hash.cfg file, run [eclint] fix to apply the style.

eclint is also added in the commit hooks (that one can install with `npm install` from the root of this repository)

[editorconfig]: https://editorconfig.org/
[eclint]: https://github.com/jedmao/eclint

See merge request nexedi/slapos!757