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

Previously you could only mark `-`-type TODOs as [Complete, Partially
Complete, Deleted]. Now you can mark any type of TODO (`-`, `_`, `!`,
`x`, `S`, `v`) as any of the above three states.

When invoking the <leader>R command to remove old todos, only remove
completed and deleted ones. Partially completed tasks should stick
around and be reverted to uncomplete.

Don't highlight matches from our `:substitute` calls. It looks weird
when search matches are highlighted but the user didn't explicitly
initiate a search.

Create some useful mappings for creating and marking TODO items. These
will be buffer-local to *.todo files, which is why we're defining them
in the ftplugin file.