Add support for non-blog websites

[Federico Ponzi]

It should support creations of blog and custom websites. I'm planning to use it for fponzi.me and blog.fponzi.me for example. They will share the same template.

In my case, both will need to generate an index.html because they're on different domains. For this reason, I'll need to add support of a config entry to specify if we want to use the template to generate a blog or an index page.

it's a bit annoying because I would like to support both cases of:

1. two projects, one template used to generate a blog and a website
2. one project, one template used to generate a blog and a website

I'd like to achieve this with the minimum additional config. This is the plan:

all blog entries are inside the content/blog folder. If there is a blog folder, then assume user wants to generate a blog.
the config will specify the index page to use to hold the blog index.

blog:
    base_template: index.html # default 
    index_destination: index.html # default 

if there is an index.md file, it means the user wants to generate a custom website. If there is an index.md file, then the user needs to change the index_destination value for the blog in the config.