Backticks are used to refer to Vim commands in help files. The Vim docs
explain:

> When referring to a Vim command and to create a hot-link, place the
> name between two backticks, eg. inside `:filetype`.  You will see this
> is highlighted as a command, like a code block (see below).

These aren't Vim commands, so use a different surrounding mark. Settled
on double quotes.

This also fixes a display problem with the "`- `" text, which was
rendered as "`- " (without the final backtick) in the highlighted help
due to the parsing for syntax highlight concealing.

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`.