Adds a 'fileContent' template function to insert the literal contents
of a file specified in the template.
Signed-off-by: Niklas Wik <niklas.wik@nokia.com>
This introduces support for looking up secret values in the 'pass' command line
tool (https://www.passwordstore.org/).
Values like passwords can be interpolated from pass and even more complex
structures like certificates for Kubernetes Secrets can be retrieved and base64-
encoded as necessary.
Fixes#2
