Alarm / Threshold updates

Maybe users don't understand the current Loss/PredictAR based alarms, they also want to be able to set their own target range instead of the default 80-180.

This PR aims to make the target range and alarm types configurable, this should address most of #271.  To do this 5 new ENV variables have been added.

   * `BG_HIGH` - default: **240**, the high BG outside the target range that is considered urgent
   * `BG_TARGET_TOP` - default: **180**, the top of the target range, also used to draw the line on the chart
   * `BG_TARGET_BOTTOM` - default: **80**, the bottom of the target range, also used to draw the line on the chart
   * `BG_LOW` - default: **55**, the low BG outside the target range that is considered urgent
   * `ALARM_TYPES` - default to **simple** if any `BG_`* ENV's are set, otherwise default to **predict**.  There currently 2 alarm types are supported, and can be used independently are separately.  The `simple` alarm type only compares the current BG to `BG_` thresholds above, the `predict` alarm type works as before by using a highly tuned formula that forecasts where the BG is going based on it's trend. 

Let's not conflate urgent and high.
This should be a high alarm type not urgent, right?