The existing `[u` and `]u` bindings allow moving to the previous and
next unfinished to-do, but when there are many low priority to-dos, and
a handful of regular and high priority items between them, those
commands are ineffective for quickly moving to the relevant ones.

Add two new sets of commands:

* `[o` & `]o` to move between regular priority to-dos
* `[y` & `]y` to move between high priority to-dos

I was looking for bracket mappings that aren't bound by default. Using
`o` seems to work, meaning "ordinary". And `y` could refer to
"yimportant". Close enough.

Allow the leader to be different for the todo filetype.

These conditions were wrong, and should have been AND-ed. Otherwise, if
you, say, didn't have a mapping to `<Plug>TwodoNewTodoBelow` and did
have a mapping to `<Leader>n` (or vice versa), the plugin would still
remap `<Leader>n`.

Add a license header to each source file, along with the appropriate
copyright years.

Our substitution commands would save the search pattern into search
history, intervening with user search history. Use the `keeppatterns`
command to prevent this.

I put these in the normal mode conditions for ease of testing and
implementation. Now check the mappings properly.

I don't really use visual mode, and don't feel like trying to make these
work.

The xmaps don't really work. Going to the next/previous TODO works, but
extending the selection doesn't because of the way I set it up with the
mark system. We'd probably need a more involved function here to do what
we want.

Currently doesn't support counts. I'll have to add that in.