Hangouts help "page" and little bugfix

[hobbypunk90]

Description:

Add a intent HangoutsHelp which returns a list of all sentences for the given conversation.
if no HangoutsHelp intent is defined and there are intents for hangouts, an entry with the sentence HELP will be generated.
Add a new config parameter default_conversations, if it's not set the intents without conversations checked in every conversation. With default_conversations set only this conversations will checked, if the conversations in the intents are not set.
Removes the tailing empty new line in messages.

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#6206

Example entry for configuration.yaml (if applicable):

hangouts:
  intents:
    HangoutsHelp:
      sentences:
        - Help
      conversations:
        - id: <id1>
        - id: <id2>
    Ping:
      sentences:
        - ping
    Marco:
      sentences:
        - marco
      conversations:
        - id: <id2>
  default_conversations:
    - id: <id1>

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.