Setup DependaBot for Charts dependency updates

[cchaos]

@chandlerprall we talked about adding RenovateBot (used in kibana) or DependaBot (github owned) few weeks ago and I remember that you prefer first update manually all your dependencies and then configure a bot for the following updates.
We are testing dependabot in elastic-charts, and this will automatically update EUI in elastic-charts. You can configure the same in EUI, so you will get a PR for every dependency that can be updated

Originally posted by @markov00 in #2572 (comment)

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