adds tests (#3)

## non-test commits
- fbcb3ab: *allow the Config to be cloned*
- 9be5ceb: *give a &Key to transition_state*
- ce401b7: *implement Default for Mode and State*
- 0770cca: *refactor the state initialization in `from_value`*
- ae3ed68: *fix overflow panic when moving in the data*
- df49a86: *make `State::from_value` public to super*
- 4c26453: *derive PartialEq and Debug for the whole config*

## TODO
- [x] `navigation.rs`: make sure the navigation in the data is ok
- [x] `app.rs`: make sure the application state machine works
- [x] `tui.rs`: make sure the rendering works as intended
- [x] `config/`: make sure the parsing of the config works

README.md

@@ -114,11 +114,11 @@ cargo doc --document-private-items --no-deps --open
 - [ ] add check for the config to make sure it's valid
 - [ ] when going into a file or URL, open it
 - [ ] give different colors to names and type
-- [ ] add tests...
-  - [ ] to `navigation.rs` to make sure the navigation in the data is ok
-  - [ ] to `app.rs` to make sure the application state machine works
-  - [ ] to `parsing.rs` to make sure the parsing of the config works
-  - [ ] to `tui.rs` to make sure the rendering works as intended
+- [x] add tests...
+  - [x] to `navigation.rs` to make sure the navigation in the data is ok
+  - [x] to `app.rs` to make sure the application state machine works
+  - [x] to `parsing.rs` to make sure the parsing of the config works
+  - [x] to `tui.rs` to make sure the rendering works as intended
 - [ ] restrict the visibility of objects when possible
 - [ ] show true tables as such