Support plugin disabled = true in qlty.toml

[brynary]

In a qlty.toml file, support the following syntax:

[[plugin]]
name = "bandit"
version = "1.7.9"
disabled = true

If a plugin is set to disabled = true, it should not be run. It should behave as if the plugin wasn't in the qlty.toml

There are two advantages to this:

1. Quickly testing disabling a plugin without removing the configuration content for the plugin
2. Telling the system that a plugin is unwanted, which allows us to avoid suggesting that plugin again