YAML isn't part of the Python standard library, while JSON is.
If we want `setup.py` to dynamically parse the available integration
manifest files in order to populate the extra dependencies, then it's
better to rely on a JSON format for manifest files - the parser is part
of the standard library and it doesn't require the user to install
`pyyaml` before `platypush`.
1. `chat.telegram` -> `telegram` plugin.
2. Merged `backend.chat.telegram` logic into `telegram` plugin.
3. Rewritten the architecture of the integration to adapt to the new
asyncio API introduced in the latest versions of telegram-bot-api.
Closes: #349