Configurable behavior for missing template vars

Initial take at functionality described in https://github.com/davidsantiago/stencil/issues/21

- Added the ability to add stencil configuration as keyword args
  - signatures changed for `render`, `render-string` and `render-file`
- Added the configuration option `:replace-missing-vars` 
  - boolean flag to determine whether to replace *missed* vars, defaults to true.