String replacement templates in commands

[dbaines]

Added string replacement for user/host/path to dynamically generate commands from existing config.

Example usage:

staging:
  host: example.com
  user: admin
  path: /data/example
  commands:
    client:
      getconfig: scp {{user}}@{{host}}:{{path}}/current/config/application.yml config/application.yml

This results in the command getting built as scp admin@example.com/data/example/current/config/application.yml config/application.yml

Feel free to suggest different string replacement template, I couldn't find any best practices for it in ruby/rails and I'm not marries to the handlebars style template.