Add function to calculate a unique base36 string for each item in a list.

The base36 string is either 3 or 4 characters large.
Add first support for todo items with a base36 ID.
Add ability to identify tasks based on their UID.
Let ListCommand print UIDs.
Surround todo identifier with bars in output.
Print todo identifiers with delete or do subcommands.

When the linenumber based todos are used, you still risk the wrong
ID is printed. But now you can used a UID to reduce the chance
significantly.