Allow to customize user functions in rule error-strings (#703)

* Allow to customize user functions in rule `error-strings`

* Rollback the Available Rules table format in README

* adds memoization of the rule's configuration

Co-authored-by: chavacava <salvadorcavadini+github@gmail.com>

testdata/error-strings-with-custom-functions.go

@@ -0,0 +1,12 @@
+package fixtures
+
+import (
+	pkgErrors "github.com/pkg/errors"
+)
+
+// Check for the error strings themselves.
+
+func errorsStrings(x int) error {
+	var err error
+	return pkgErrors.Wrap(err, "This %d is too low") // MATCH /error strings should not be capitalized or end with punctuation or a newline/
+}