refactor: build generators from data (don't use subclasses)

This moves `id`, `fileExtension`, and `displayName` from per-subclass
virtual overrides to base-class members set through the constructor.

That's a prerequisite for letting users add a Handlebars-based generator
without writing C++.

Author: gennaroprota

src/lib/Gen/adoc/AdocGenerator.cpp

@@ -11,11 +11,17 @@
 
 #include "AdocGenerator.hpp"
 #include "AdocEscape.hpp"
-#include <mrdocs/Support/Handlebars.hpp>
+#include <memory>
 
 namespace mrdocs {
 namespace adoc {
 
+AdocGenerator::
+AdocGenerator()
+    : HandlebarsGenerator("adoc", "adoc", "Asciidoc")
+{
+}
+
 void
 AdocGenerator::
 escape(OutputRef& os, std::string_view const str) const

src/lib/Gen/adoc/AdocGenerator.hpp

@@ -13,34 +13,15 @@
 #ifndef MRDOCS_LIB_GEN_ADOC_ADOCGENERATOR_HPP
 #define MRDOCS_LIB_GEN_ADOC_ADOCGENERATOR_HPP
 
-#include <mrdocs/Platform.hpp>
 #include <lib/Gen/hbs/HandlebarsGenerator.hpp>
-#include <mrdocs/Generator.hpp>
 
 namespace mrdocs::adoc {
 
 class AdocGenerator final
     : public hbs::HandlebarsGenerator
 {
 public:
-    std::string_view
-    id() const noexcept override
-    {
-        return "adoc";
-    }
-
-    std::string_view
-    fileExtension() const noexcept override
-    {
-        return "adoc";
-    }
-
-
-    std::string_view
-    displayName() const noexcept override
-    {
-        return "Asciidoc";
-    }
+    AdocGenerator();
 
     void
     escape(OutputRef& os, std::string_view str) const override;

src/lib/Gen/html/HTMLGenerator.cpp

@@ -11,10 +11,17 @@
 
 #include "HTMLGenerator.hpp"
 #include <mrdocs/Support/Handlebars.hpp>
+#include <memory>
 
 namespace mrdocs {
 namespace html {
 
+HTMLGenerator::
+HTMLGenerator()
+    : HandlebarsGenerator("html", "html", "HTML")
+{
+}
+
 void
 HTMLGenerator::
 escape(OutputRef& os, std::string_view str) const

src/lib/Gen/html/HTMLGenerator.hpp

@@ -23,23 +23,7 @@ class HTMLGenerator final
     : public hbs::HandlebarsGenerator
 {
 public:
-    std::string_view
-    id() const noexcept override
-    {
-        return "html";
-    }
-
-    std::string_view
-    fileExtension() const noexcept override
-    {
-        return "html";
-    }
-
-    std::string_view
-    displayName() const noexcept override
-    {
-        return "HTML";
-    }
+    HTMLGenerator();
 
     void
     escape(OutputRef& os, std::string_view str) const override;