Implement Page bundling and image handling

This commit also improves on the following:

* Symbolic links are now allowed anywhere inside /content

Fixes gohugoio#3651
Closes gohugoio#3158
Fixes gohugoio#1014
Closes gohugoio#2021
Fixes gohugoio#1240
Updates gohugoio#3757

Author: bep

Gopkg.lock

@@ -21,8 +21,8 @@
  revision = "v1.1.0"
 
 [[constraint]]
-  branch = "master"
-  name = "github.com/dchest/cssmin"
+ name = "github.com/disintegration/imaging"
+ revision = "v1.2.4"
 
 [[constraint]]
   name = "github.com/magefile/mage"
@@ -116,6 +116,10 @@
   name = "github.com/stretchr/testify"
   version = "1.1.4"
 
+[[constraint]]
+  branch = "master"
+  name = "github.com/olekukonko/tablewriter"
+
 [[constraint]]
   name = "github.com/yosssi/ace"
   version = "0.0.5"