Support config file for arguments like the app-id

[miquelbeltran]

At the moment, all parameters must be provided via arguments. Would be good to be able to read from a "dotfile" in the project root or similar.

e.g. a file named .raygun with the configuration for the raygun-cli.

[miquelbeltran]

Idea, load from .env type files e.g. https://pub.dev/packages/dotenv

[TheRealAgentK]

So, there are a few things I noticed with #6 that would probably also apply to this.

Not all the various Raygun functionality seems to use the new API yet and the same ways of accessing features.

Currently app-id and token (this would be a Personal Access token) are configured via Config_props.

The new deployments command I added in #7 however, doesn't need an app-id at all, so I shouldn't even have to think about supplying it.

The new proguard command I'm adding in #8 needs an app-id, but it's not on the v3 API yet, I need to use an Extended Access Token, with is different from the token from Config_props. The command has external-access-token via the CLI, but if I want to use app-id from Config_props I also have to supply a token in there.

Not sure how to best deal with these different ways to authenticate in a clean way. It seems to me that everything you'd supply in Config_props needs to be optional.

[miquelbeltran]

Makes sense, ConfigProps was created with the symbols and sourcemap commands in mind, but we can always add new or make parameters configurable per command, or maybe create a different ConfigProps for those commands that don't support appid + token.

[TheRealAgentK]

Note: My comment above was fixed in #15