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.

`[u` and `]u` for "unfinished".

Define the `todo#Escalate()` function which does the opposite of
`todo#Descalate()` and add a mapping to invoke the function.