Ingress, Custom Resources, Watch API and Kubernetes v1.18

New features:

• Support for Ingress API [#19 by @novakov-alexey]
• Support for Custom Resources API [#16 by @novakov-alexey]
• Support for Watch API [#12 by @novakov-alexey]
• Upgrade to Kubernetes v1.18 [#12 by @novakov-alexey]

Other:

• Switch build to Mill [#17 by @joan38]
• Tests against Kubernetes 1.18, 1.17 and 1.16 [#13 by @joan38]