adding initContainers and hostAliases

New Features:

• Add support for injecting initContainers! #8 Thanks to @iwilltry42 and @zach-dunton-sf!
• Add support for injecting hostAliases! #5 Thanks to @iwilltry42

See https://github.com/tumblr/k8s-sidecar-injector/blob/master/docs/sidecar-configuration-format.md for the full sidecar config format and examples of the new injections

Bugfixes:

• Fix volumemount injection not correctly identifying duplicate volumemounts #6 - thanks @bnadim!

Changes since last release: v0.1.2...v0.1.4