Config Validation

## Is your feature request related to a problem? Please describe
There currently isn't any validation performed on the config file during the load. Adding such validation would provide some assurance that there aren't any configuration errors, for example, nob-existent word templates.

## Describe the solution you'd like
Build a validation engine with [Fluent Validations](https://fluentvalidation.net/). Also, defining an XML schema would also improve the developer experience.