Easy to add a priority to the current line

- several new leader keys to add priority to the current line
- updated README
Easy to add a priority to the current line

- several new leader keys to add priority to the current line
- updated README
Merge branch 'dev-add-priority' of https://github.com/ha17/todo.txt-vim into dev-add-priority
Merge branch 'dev-add-priority' of https://github.com/ha17/todo.txt-vim into ha17-dev-add-priority
Updated documentation and minor fixes.
Merge branch 'ha17-dev-add-priority'