Fix RelPermalink() and Urls in menus vs canonifyUrls

canonifyUrls=true, RelPermalink and baseUrl with sub-path did not work.

This fixes that by adding a check for canonifyUrl=trues=true in RelPermalink().

So given

- baseUrl "http://somehost.com/sub/"
- the path "some-path/file.html"

For canonifyUrls=false RelPermalink() returns "/sub/some-path/file.html"
For canonifyUrls=true RelPermalink() returns "/some-path/file.html"

In the last case, the Url will be made absolute and clickable in a later step.

This commit also makes the menu urls defined in site config releative. To make them work with canonifying of urls, the context root is prepended if canonifying is turned off.

Fixes #519
Fixes #711

docs/content/extras/menus.md

@@ -96,10 +96,12 @@ Here’s an example `config.toml`:
         pre = "<i class='fa fa-heart'></i>"
         weight = -110
         identifier = "about"
+        url = "/about/"
     [[menu.main]]
         name = "getting started"
         pre = "<i class='fa fa-road'></i>"
         weight = -100
+        url = "/getting-started/"
 
 And the equivalent example `config.yaml`:
 
@@ -110,11 +112,16 @@ And the equivalent example `config.yaml`:
             Pre: "<i class='fa fa-heart'></i>"
             Weight: -110
             Identifier: "about"
+            Url: "/about/"
           - Name: "getting started"
             Pre: "<i class='fa fa-road'></i>"
             Weight: -100
+            Url: "/getting-started/"
     ---            
 
+
+**NOTE:** The urls must be relative to the context root. If the `BaseUrl` is `http://example.com/mysite/`, then the urls in the menu must not include the context root `mysite`. 
+  
 ## Nesting
 
 All nesting of content is done via the `parent` field.