* Add index parameter to scrape sensor
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.
* Set default value for index to avoid later checks
David Barrera