feat: add links to rules in formatters

This PR introduces:

- A new formatter called "plain" which produces the same output as "default" but also appends link to the rule
- The "friendly" and "stylish" formatters now have URLs to the rules instead of the rule names
- Update of the README, adding the "unix" and "plain" formatters

formatter/plain.go

@@ -0,0 +1,26 @@
+package formatter
+
+import (
+	"fmt"
+
+	"github.com/mgechev/revive/lint"
+)
+
+// Plain is an implementation of the Formatter interface
+// which formats the errors to JSON.
+type Plain struct {
+	Metadata lint.FormatterMetadata
+}
+
+// Name returns the name of the formatter
+func (f *Plain) Name() string {
+	return "plain"
+}
+
+// Format formats the failures gotten from the lint.
+func (f *Plain) Format(failures <-chan lint.Failure, _ lint.RulesConfig) (string, error) {
+	for failure := range failures {
+		fmt.Printf("%v: %s %s\n", failure.Position.Start, failure.Failure, "https://revive.run/r#"+failure.RuleName)
+	}
+	return "", nil
+}