Add supported_platforms for plugins

[brynary]

Certain plugins may not be compatible with all platforms that we run on. For example, we have identified issues with Hadolint on MacOS, and we have identified issues with swiftlint on non-MacOS systems.

Add a supported_platforms array key to the plugin definition:

[plugins.definitions.swiftlint]
supported_platforms = ["macos"]
# ...

Valid array values are "macos", "linux", and "windows".

If supported_platforms is empty, treat it as all platforms. If supported_platforms is not blank:

1. qlty init should only enable the plugin if it is on a supported platform.
2. The planner should skip the plugin when running on a supported platform (if the platform is unsupported on the current platform, log an info!)