Added `do` command
Add searching in `list`
Added `task_attr` helper, `listall` implementation, and negative search terms
Use trailing - for negative rather than leading (since that makes it a command-line option)
Case-insensitive search
Added `listcon` method
Added search for projects, and print out tasks after adding them
Added priority searching; set undefined priority by default, and define how todo.txt priorities map to caldav ones
Added `prepend` command
Use priority map to format task, and adjust order of text
Distinguish between missing and multiple keys
Added `pri` command, and make `depri` set a priority of 0 (equivalent to 5, but means non-prioritized)