Elmstatic 0.3.1: generate subsections, make index pages, add page aliases

New release is out today. Now it allows you to:

Create subsections with their own posts
Create Index.elm pages which turn into index pages, both at the root level and at subsection level. For example, Pages/Index.elm becomes the page at / in contrast to Pages/Projects.elm which becomes /projects. At subsection level, Pages/Elm/Index.elm becomes the page at /elm, while Pages/Elm/Projects.elm becomes /elm/projects.
Use the postprocessing copy step to give pages aliases (eg /projects can be made ...