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.

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

Commands should be able to be repeated. Makes it easier to mark several
to-dos complete in a row, for example.

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

Learned that `search()` accepts an `s` flag to mark the previous cursor
location, so figured we should take advantage of that and remove the
manual mark line.

`[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.