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/permalinks_test.go

@@ -0,0 +1,75 @@
+package hugolib
+
+import (
+	"strings"
+	"testing"
+)
+
+// testdataPermalinks is used by a couple of tests; the expandsTo content is
+// subject to the data in SIMPLE_PAGE_JSON.
+var testdataPermalinks = []struct {
+	spec      string
+	valid     bool
+	expandsTo string
+}{
+	{"/:year/:month/:title/", true, "/2012/04/spf13-vim-3.0-release-and-new-website/"},
+	{"/:title", true, "/spf13-vim-3.0-release-and-new-website"},
+	{":title", false, ""},
+	{"/blog/:year/:yearday/:title", true, "/blog/2012/97/spf13-vim-3.0-release-and-new-website"},
+	{":fred", false, ""},
+	{"/blog/:fred", false, ""},
+	{"/:year//:title", false, ""},
+	{
+		"/:section/:year/:month/:day/:weekdayname/:yearday/:title",
+		true,
+		"/blue/2012/04/06/Friday/97/spf13-vim-3.0-release-and-new-website",
+	},
+	{
+		"/:weekday/:weekdayname/:month/:monthname",
+		true,
+		"/5/Friday/04/April",
+	},
+	{
+		"/:slug/:title",
+		true,
+		"/spf13-vim-3-0-release-and-new-website/spf13-vim-3.0-release-and-new-website",
+	},
+}
+
+func TestPermalinkValidation(t *testing.T) {
+	for _, item := range testdataPermalinks {
+		pp := PathPattern(item.spec)
+		have := pp.validate()
+		if have == item.valid {
+			continue
+		}
+		var howBad string
+		if have {
+			howBad = "validates but should not have"
+		} else {
+			howBad = "should have validated but did not"
+		}
+		t.Errorf("permlink spec %q %s", item.spec, howBad)
+	}
+}
+
+func TestPermalinkExpansion(t *testing.T) {
+	page, err := ReadFrom(strings.NewReader(SIMPLE_PAGE_JSON), "blue/test-page.md")
+	if err != nil {
+		t.Fatalf("failed before we began, could not parse SIMPLE_PAGE_JSON: %s", err)
+	}
+	for _, item := range testdataPermalinks {
+		if !item.valid {
+			continue
+		}
+		pp := PathPattern(item.spec)
+		result, err := pp.Expand(page)
+		if err != nil {
+			t.Errorf("failed to expand page: %s", err)
+			continue
+		}
+		if result != item.expandsTo {
+			t.Errorf("expansion mismatch!\n\tExpected: %q\n\tReceived: %q", item.expandsTo, result)
+		}
+	}
+}