Add a `yorm.fake` setting to bypass the filesystem

When other libraries are using YORM, they probably won't want it creating and modifying files during their tests:

- [ ] create a `yorm.fake` boolean that defaults to false
- [ ] add a `yorm_text` attribute to mapped objects that contains the text
- [ ] if `yorm.fake` is set, `yorm_text` should not be:
  - [ ] written to the filesystem
  - [ ] read from the filesystem