Add a mechanism to make Topiary generate its default configuration

[Niols]

Is your feature request related to a problem? Please describe.

Now that Topiary can get its configuration from external files, it would be nice if there was a possibility to generate the default configuration it uses to a certain file.

Describe the solution you'd like

A sub-command and/or flag to make Topiary output its default (or maybe current if there are other configuration files at play?) configuration to stdout or to a file.

Describe alternatives you've considered

A cheap alternative would be to ship the default configuration file in Topiary, eg. in share/doc/language.toml or something, and to advertise that somewhere for someone (such as me) looking for it.

Additional context

Issues:

• Configure the language(s) in the pre-commit hook #447
• Add a mechanism to provide configuration from a chosen place #564
  talk about things related to this.

In particular, one property should probably be that generating the current configuration then using it gives the same behaviour (more idempotency yay \o)

[Xophmeister]

This is implemented in #583