Add current balance to hydroquebec sensor

[titilambert]

Description:

Add current balance to Hydroquebec sensor

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#2861

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: hydroquebec
    username: MYUSERNAME
    password: MYPASSWORD
    contract: '123456789'
    monitored_variables:
     - balance
     - period_total_bill
     ...

Checklist:

• Documentation added/updated in home-assistant.github.io
• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.

[fabaff]

Thanks 🐦