Move sensible-TYPE to default lists.

The sensible-browser, sensible-editor, and sensible-pager commands
weren't special and can just be treated identically to the other
commands.  This is a small BC-break (anyone overriding the commands will
get different behavior now), but it's worth it to clean up the API.
Require usage of nubs/which locators.

These locators make things so much simpler to reason with and improve
usage all around.  The nicety around not having an external dependency
doesn't make a big enough difference when the functionality it provides
is so useful.

Perhaps in the future this dependency can be removed and made optional
again.
Return null when no command found.

The previous was returning the very last command.  It's better to just
return null since the default won't work anyhow.
Specify commands by basename rather than absolute path.

The absolute path isn't useful anymore.  The user's PATH will be much
better at locating the commands.
Move paths out of the options array.

The paths are a pretty standard part of these commands.  They don't
belong hidden away down in an options array and should be moved to the
forefront.
Move habitat environment parameter out of options array.

There is no need to have an options array for only one option with no
additional options on the horizon.