Export directly to a file, plus use the prefix argument to read the current buffer

This release includes:

• CSV output to a file (as opposed to a buffer) with org-clock-csv-to-file. Suggested by @milouse.
• Rendering entries from the current buffer by using the prefix arg to either org-clock-csv or the new org-clock-csv-to-file. Suggested by @quazgar.
• Cask support. Contributed by @Fuco1.
• Support for duration and headline in custom formatters. Contributed by @milouse.

In addition to better testing and other minor fixes and improvements.