Update weatheralerts_2.yaml package

This update adds icons for each alert type (Issue #50) and should take care of most, if not all, of the template errors that were getting logged during startup and each time the sensor became unavailable due to outages and other problems with the weather.gov alerts API (Issue #52).

Fix for no 'version' key issue #43.

Fix for issue #43. Add version key to manifest and update version number.

Update CHANGELOG.md

Fix links in TROUBLESHOOTING.md

Fix ordering with simultaneous alerts

Sometimes more than one alert is issued with the same 'sent' date/time. The alert json feed randomly changes the ordering of these alerts in the feed and can result in re-triggering of notifications. The alerts array is now being reverse sorted by 'sent' time and 'id' to keep them consistent when the json feed shuffles them around.

Add county ID, additional alert properties, and PlatformNotReady (#29)

No breaking changes. If you have a weatheralerts platform configuration already set, it should continue to work. However, for alert reliability, it is highly recommended that you add the new 'county' platform config option to specify your county ID number (it is not likely going to be the same number as the zone ID number). Check the README.md documentation for details.

Added 'county' platform config option to specify an optional county ID number. 'zone' should be set to a proper zone ID number (and is required), the county ID number should then be specified with the 'county' option.

Additional alert properties have been added along with proper handling in the event an attribute is not included in the json alert feed. Feel free to request additional alert properties by submitting an issue here on github (https://github.com/custom-components/weatheralerts/issues).

The main platform sensor entity state will contain the number of active alerts, unless the sensor fails to connect with weather.gov to retrieve alerts. If the sensor cannot connect to weather.gov to retrieve the json alert feed, the sensor state will be set to 'unavailable'. The yaml package(s) with this pull request includes template sensors that will retain active alerts and the active alert count (for a short period), even when the main sensor becomes 'unavailable'.

PlatformNotReady is added to allow the platform to continue to retry the initialization if the weather.gov site is too slow to respond or experiencing other issues.

Added additional checks to verify the specified zone and county IDs are valid. 

Included a set of comprehensive yaml packages that include tracking for up to 5 active weather alerts and automations and script to handle UI notifications.

Started update of README.md documentation.