Add index parameter to scrape sensor

[davidbb]

Description:

The scrape sensor selects the first element of the list returned by
BeautifulSoup. This commit adds an optional index parameter to allow the
selection of a different element from the list of results. To make this
a non-breaking change, if no index value is configured, the sensor
defaults to the previous behaviour of returning the first element.

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#8590

Example entry for configuration.yaml (if applicable):

sensor:
 - platform: scrape
   name: h-index
   resource: https://scholar.google.ca/citations?user=cVeVZ1YAAAAJ
   select: "td:is(.gsc_rsb_std)"
   index: 2

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

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

• Documentation added/updated in home-assistant.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.

[homeassistant]

Hi @davidbb,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!

[fabaff]

Thanks 🐦