As I said in 095fb9fef4, removing support for `~/.jjconfig` was an
experiment. I've heard from a few people (including in #233) that they
would prefer to have configs in the home directory. This patch
therefore restores that functionality, except I added a `.toml`
extension to the file to clarify the expected format to users and
editors.
After this patch, we still allow configs in `$XDG_CONFIG_HOME` (and
the other paths used by `dirs::config_dir()`), but we error out there
are config files in both that location and `~/.jjconfig.toml`.