Add bump:nodejs task

[ybiquitous]

This change also adds a Aufgaben::Bump::Base class where the common bumping is written.

Example:

require "aufgaben/bump/nodejs"
Aufgaben::Bump::Nodejs.new do |t|
  t.files << "Dockerfile.production" # defaults to ["Dockerfile"]
  t.version_files << "nodejs-version.txt" # defaults to [".node-version", ".nvmrc"]
end

• .node-version file is supported by nodenv.
• .nvmrc file is supported by nvm.