This PR adds granular configuration options to specify what is required, forbidden or ignored.

Breaking Changes

• The configuration format has changed, the .lintspec.toml files from v0.3 are not compatible. A new lintspec init command is available to generate a fresh default config file.
• Validation defaults: there are new defaults for which diagnostics are emitted by default. Review the new defaults by generating a config file with lintspec init.
• The CLI arguments (config::Args) have changed, there is no --constructor, --struct-params, --enum-params, --enforce, --enforce-all anymore. They have been replaced with new args, see documentation.
• The fields in config::Config have changed drastically
• The definitions::ItemType enum now has more members
• The definitions::SourceItem trait now takes in &self for item_type(&self).
• config::read_config now takes the parsed CLI args as argument.
• lint::ValidationOptions has changed drastically
• lint::{check_params, check_returns} have new arguments.

Closes #55