Add a --markdown flag to GN to be able to output help in markdown.

With this change, we can now generate a markdown page containing all
of the built-in help from GN, so that it will be visible on the web.

The changes to GN to support markdown are, so far, minimal, but the
resulting format is also pretty lame: we just wrap the body of the each
help command in a code block (markdown's equivalent of <pre>).

R=brettw@chromium.org
BUG=468851

Review URL: https://codereview.chromium.org/1024783002

Cr-Commit-Position: refs/heads/master@{#325730}

tools/gn/README.md

@@ -94,6 +94,7 @@ Read these links:
   * [FAQ](docs/faq.md)
   * [GYP conversion cookbook](docs/cookbook.md)
   * [Language and operation details](docs/language.md)
+  * [Reference](docs/reference.md) The built-in `gn help` documentation.
   * [Style guide](docs/style_guide.md)
   * [Cross compiling and toolchains](docs/cross_compiles.md)
   * [Hacking on GN itself](docs/hacking.md)

tools/gn/command_help.cc

@@ -69,18 +69,35 @@ void PrintToplevelHelp() {
 }
 
 void PrintSwitchHelp() {
+  const base::CommandLine* cmdline = base::CommandLine::ForCurrentProcess();
+  bool use_markdown = cmdline->HasSwitch(switches::kMarkdown);
+
   OutputString("Available global switches\n", DECORATION_YELLOW);
   OutputString(
       "  Do \"gn help --the_switch_you_want_help_on\" for more. Individual\n"
       "  commands may take command-specific switches not listed here. See the\n"
       "  help on your specific command for more.\n\n");
 
+  if (use_markdown)
+    OutputString("```\n\n", DECORATION_NONE);
+
   for (const auto& s : switches::GetSwitches())
     PrintShortHelp(s.second.short_help);
+
+  if (use_markdown)
+    OutputString("\n```\n", DECORATION_NONE);
 }
 
 void PrintAllHelp() {
-  PrintToplevelHelp();
+  const base::CommandLine* cmdline = base::CommandLine::ForCurrentProcess();
+  if (cmdline->HasSwitch(switches::kMarkdown)) {
+    OutputString("# GN Reference\n\n");
+    OutputString("[TOC]\n\n");
+    OutputString("*This page is automatically generated from* "
+                 "`gn help --markdown all`.\n\n");
+  } else {
+    PrintToplevelHelp();
+  }
 
   for (const auto& s : switches::GetSwitches())
     PrintLongHelp(s.second.long_help);