Add an option to allow change "to" param to 'now' instead of the end time of a shift

[hungnt61h]

There is a problem that when I enabled the Real-time shift auto-select option, the "to" parameter on url was sought to the end time of a shift. That was not really helpful while I was querying PromQL that calculated a sum of data point in a time range like from the start time of the shift till now. To solve this problem, I have modified your code to add a new option like below.

So my query is able to calculate the sum of data points from the start time of a shift till present.

I hope you would approve this feature and guide me how to contribute my work to your plugin.
By the way, I would suggest another feature that is adding a shift countdown. It would appear below the shift selector on the same panel.

[hungnt61h]

@isaozler Could you review my PR #39 ? Could we discuss more about this?

[hungnt61h]

The main reason behinding this feature is that I need the shift changes automatically, but the calculation of the time range which the plugin overrides does not work with Prometheus. PromQL cannot limit data between 2 points, one from the past and one from the future. It can only calculate a metric limited by a point in the past and now or 2 points in the past.