More seamless path

[sushain97]

• [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