configurable permalinks support

A sample config.yaml for a site might contain:

```yaml
permalinks:
  post: /:year/:month/:title/
```

Then, any article in the `post` section, will have the canonical URL
formed via the permalink specification given.

Signed-off-by: Noah Campbell <noahcampbell@gmail.com>

hugolib/config.go

@@ -34,6 +34,7 @@ type Config struct {
 	Indexes                                    map[string]string // singular, plural
 	ProcessFilters                             map[string][]string
 	Params                                     map[string]interface{}
+	Permalinks                                 PermalinkOverrides
 	BuildDrafts, UglyUrls, Verbose             bool
 }
 
@@ -70,6 +71,11 @@ func SetupConfig(cfgfile *string, path *string) *Config {
 		c.Indexes["category"] = "categories"
 	}
 
+	// ensure map exists, albeit empty
+	if c.Permalinks == nil {
+		c.Permalinks = make(PermalinkOverrides, 0)
+	}
+
 	if !strings.HasSuffix(c.BaseUrl, "/") {
 		c.BaseUrl = c.BaseUrl + "/"
 	}