Do not create robots.txt by default

Meny people, including me, have a custom robots.txt in static.

Also remove that option from the command line; it doesn't feel
important enough.

Fixes ##2049

docs/content/extras/robots-txt.md

@@ -13,13 +13,12 @@ weight: 120
 Hugo can generated a customized [robots.txt](http://www.robotstxt.org/) in the
 [same way as any other templates]({{< ref "templates/go-templates.md" >}}).
 
-By default, it generates a robots.txt, which allows everything, with the following content:
+To enable it, just set `enableRobotsTXT` option to `true` in the [configuration file]({{< ref "overview/configuration.md" >}}). By default, it generates a robots.txt, which allows everything, with the following content:
 
 ```http
 User-agent: *
 ```
 
-To disable it, just set `disableRobotsTXT` option to `false` in the [command line]({{< ref "commands/hugo.md" >}}) or [configuration file]({{< ref "overview/configuration.md" >}}).
 
 Hugo will use the template `robots.txt` according to the following list in descending precedence:
 

docs/content/overview/configuration.md

@@ -95,8 +95,8 @@ Following is a list of Hugo-defined variables that you can configure and their c
     disableRSS:                 false
     # Do not build Sitemap file
     disableSitemap:             false
-    # Do not build robots.txt file
-    disableRobotsTXT:           false
+    # Build robots.txt file
+    enableRobotsTXT:           false
     # edit new content with this editor, if provided
     editor:                     ""
     # Enable Emoji emoticons support for page content.