Upgrading from 0.3.0 to 0.4.0

Some of the command configuration has changed dramatically in this release. The old run_mode and
chdir config keys have been replaced by new options, invoke, working_dir, and path_args.
The old keys have been deprecated. They will continue to work for a while but are no longer
documented.

Here is what the new config looks like for all possible combinations of the run_mode and chdir
keys.

run_mode = "files"
chdir = false

This was the default, and the equivalent defaults produce the same result:

invoke = "per-file"
working_dir = "root"
path_args = "file"

run_mode = "files"
chdir = true

invoke = "per-file"
working_dir = "dir"
path_args = "file"

run_mode = "dirs"
chdir = false

invoke = "per-dir"
working_dir = "root"
path_args = "dir"

run_mode = "dirs"
chdir = true

invoke = "per-dir"
working_dir = "dir"
path_args = "none"

run_mode = "once"
chdir = false

invoke = "once"
working_dir = "root"
path_args = "dot"

run_mode = "once"
chdir = true

invoke = "once"
working_dir = "root"
path_args = "none"

But note that the new config options allow for many other possibilities that couldn't be expressed
with the old options.
