Add docker secrets support for TELEGRAM_API_ID and TELEGRAM_API_HASH

[jieggii]

Summary

Resolves #19

After this update users will be able to provide Telegram API credentials via env vars containing paths to files containing the actual values.

Example showcase:

1. Create ./secrets dir and containing telegram_api_hash and telegram_api_id files:

    secrets
   ├──  telegram_api_hash
   └──  telegram_api_id

2. Fill the files with the actual credentials:

   $ echo "12345" > ./secrets/telegram_api_id
   $ echo "ba78...165ad" > ./secrets/telegram_api_hash

3. Create docker-compose.yml, for example:

   
   services:
   telegram-bot-api:
   build: .
   
   environment:
     # in this section we set paths to the mounted secrets: 
     TELEGRAM_API_ID_FILE: /run/secrets/telegram_api_id
     TELEGRAM_API_HASH_FILE: /run/secrets/telegram_api_hash
   
   secrets:
     # here we define list of secrets that will be used by the service
     - telegram_api_id
     - telegram_api_hash

secrets:

here we define the secrets and their location on the host machine:

telegram_api_id: file: ./secrets/telegram_api_id telegram_api_hash: file: ./secrets/telegram_api_hash

# Notes
I've also included checks for env vars presense:

$ docker run telegram-bot-api:latest error: expected TELEGRAM_API_ID or TELEGRAM_API_ID_FILE env vars to be set

I think this will slightly improve user experience and will not break anything, as environmental variables containing Telegram API credentials were essential to run the telegram-bot-api anyway.

Please share your thoughts and whether any changes needed. 
Then I will update the README file to reflect the usecase changes.