[RFC] Support Dhall format for project configuration

Currently we use TOML format for initial default config for project scaffolding. See the example of config here:

* https://github.com/kowainik/org/blob/master/.summoner.toml

But it would be nice to support Dhall as well. This might be convenient for some people.

The idea is that `summoner` will check the extension of the file and use the parser depending on the extension.

Start with the linked .summoner.toml example and trace how summoner currently loads TOML configuration. Investigate the existing configuration entry point and parser selection, then define how Dhall files should be recognized alongside TOML. Done means project scaffolding accepts Dhall configuration files without regressing TOML support.

Written by the indexing model from the issue text.