Remove Developer plugin

We don’t really need the developer plugin. It will be easier just to
manage plugins directly in Puppet.
Add Debug Bar Slow Actions

Closes #131
Simplify definition of `line`

The `onlyif` parameter is awkward because it’s really for Execs. Plus,
we don’t need it. An empty string evaluates to false in Puppet, so we
can use `if ( $empty_string )` instead.