fix commit calendar svg parsing for getting the highest number of commits

[xavierforge]

The current script is unable to capture the highest number of commits. GitHub changed the contribution calendar SVG format again after pull request #106 Fortunately, the pattern $(NUMBER OF COMMIT) contributions on $(DATE) stays the same and seems unlikely to change further. So, this pull request uses regex to get the number of commits in the function parse_contributions_calendar. Now, it can get the highest number of commits instead of 0 and generate an excellent calendar visual.