Add a custom StartDate option to trackRecord feature.

[noemiquezada]

Expanded the trackRecord() feature to allow Users to select a StartDate in which they want the trackRecord to start at. Right now it defaults to today's date, but I was hoping that it could calculate trackRecords for a specific week, starting at the end of that week. So thought, adding the custom StartDate will solve for this usecase and other similar use cases in which a User doesn't want to use today's date for the start of the TrackRecord.

Changelog:

• Accept a custom StartDate (optional) in trackRecord()
• Created tests for custom StartDate
• Created tests for sortDates in helper.js to check for empty arrays.
• Updated README.md TrackRecord section with new custom 'StartDate' option.

[noemiquezada]

I agree! endDate is a better name for the custom option. I've committed your suggestions. Looking forward to using this feature. 🙂