[Partial Feature] Table name as field instead of Pragma
## Feature Request

### Is your feature request related to a problem?
Trying to dynamically add tables to my ORM based CMS

### Describe the solution you'd like
Add a check on Model fields at **[norm/postgres.nim(134, 96)](https://github.com/moigagoo/norm/blob/767e04c877f58fcb2894f5d20322c606b6559794/src/norm/postgres.nim#L134)** to see if the object contains the field with the _key_ table, in which case we use it to create the table name

### Describe alternatives you've considered
I've considered manipulating nim AST, forking the project for my own purpose, yet I still think this could be an useful feature allowing to expand the usage of norm for web-development along with frameworks like Jester or Prologue

### Teachability, Documentation, Adoption, Migration Strategy
Norm's Users may use this feature to reduce duplicated code, keeping cleaner their work and allowing them to test more easily without having to re-build the software.

Conversion from JSON to SQL Table could be easily achieved as well wrapping Model inside a **[JsonModel](https://pastebin.com/EDj4N0Yi)**

How can norm benefit from this feature in the future? Norm could become the Nim ORM standard by _de facto_ implementing a core feature to build Search Engines and similar on top of this feature see [Elasticsearch Json Processor](https://www.elastic.co/guide/en/elasticsearch/reference/master/json-processor.html) for example