Allow running tools with default configs

[felixjung]

Right now you can bootstrap tool configurations via the bootstrap-config CLI command. This is nice, but it should only be necessary, if you have to customize a config for your project.

In order to allow users to run tools with a default configuration, we should allow them to specify which default config to use in the run command. For example, if someone runs

> sumup-toolkit run eslint --config react

We should check, if there is a default config for react and use it. Obviously, there the --config flag conflicts with the ESLint flag. However, since the run command proxies for ESLint, we have the power to determine if the value is a simple string or a path to a configuration file. This would effectively allow us to replace the value with a path to one of our default configurations.

Right now, the default configurations are exported from a simple module per tool. In order to make this work, we'd have to have multiple config files that export the respective default configs for us. Maybe we can create these configs during build using the same technique we use to create bootstrap configs.