More seamless path

- [x] Let's rename from `pairs_path` to just `path`. There don't need to be only pairs under it, there can be language modules as well.
- [ ] I should be able to adjust it like `apertium.path.extend(...)` or `apertium.path = ...`. Not an unintuitive `append_pairs_path` function. The module should keep track of what path it used for its last search and if it's different from the current one, re-search the path before checking for a mode.
- [ ] tests for the above
- [ ] update README