Add automated dependency management

[Madsn]

New feature description

renovate or greenkeeper will open issues when a dependency can/should be updated.
I've requested installation of renovate (and greenkeeper before I found renovate) for this repository already.

Additional context

See this section for details on how greenkeeper works: https://greenkeeper.io/docs.html#greenkeeper-step-by-step
Renovate does something similar, but seems to be a little less straightforward to set up - but more flexible I suppose.

[Madsn]

Other options:
snyk
dependabot

[Madsn]

A poll perhaps? (found this and thought it was worth trying out: https://gh-polls.com/)

[davefollett]

If renovate is added, make sure to add the badge to the topleve README.md.