Add support for uploading commits to fill in white (zero-commit) dates

[LuisAPI]

Rather than blindly uploading a commit every day for as long as a script runs, for a given GitHub account I may want to instead “fill in the blanks” of a GitHub contribution chart by only having it upload commits on given dates where there weren't any commits.

Expected solution

1. Ask for given user input: option to always upload commits (default) or upload only to prefill past selected dates.
2. (If 2nd option is chosen —) ask for given user input: type a number in which only the past n days will be checked for commits.
3. In the list of dates where there were zero commits found on a GitHub profile, upload a commit using the command git commit and parameter --date yyyy-mm-ddThh:mm:ss.
4. Check GitHub profile again.
5. The contribution chart in the given date range should be all green!

[RickyBhatti]

Before anything, I appreciate the in-depth post (especially since I don't have any default templates).

That's an interesting idea for sure, never really considered anyone wanting it. I'll probably look into implementing that soon, whenever I get a bit of free time.