Sigi 2.0.0

Added:

• Better description of --verbose flag in sigi --help text.
• New --noisy alias of --verbose
• Issue and wiki links in project README
• Graceful handling of non-digits in head/tail
• Faster execution when running in silent mode

Changed:

• On-disk file format has changed. (Still JSON, but slightly different structure)
• Completed/Deleted items now move to ${stack_name}_history - previously they moved to _completed and _deleted stacks.
• Time stamps now use an RFC 2822 date and time format like Tue, 1 Jul 2003 10:52:37 +0200. (Note: in the future this will be configurable)
• Lots of refactoring and tidying.