add todo-focus for entering an environment to concentrate on a subset of tasks

e.g. todo-focus @phd would enter an environment where only those tasks with
the @phd tag were listed when calling 'todo' or will be modified by 'todone'.
ignore repeated tags given on the command line

This only really matters for the basic 'todo'. Now you can do `todo @foo @bar
@foo test' and it the second @foo will be deleted. This is particularly useful
when using todo-focus, when it's easy to repeat a tag that is already implied
by the todo-focus environment.