Extensibility

This attempts to address extensibility issues raised in https://github.com/carrot/roots-cms/issues/19.

- Moves the configuration file out of the main `roots-cms` and has it live in the roots project itself (`cms.json` in the root directory)
- Adds ability to load custom templates and stylesheets for use in `roots-cms` (only in the main content show page right now) using the config file in the roots project
- Default content show template renders all the front matter data and allows updating and saving of changes

#### Roots project config

Add a `cms.json` file into the root of the project directory. An example config file looks like this:

```json
{
  "content_dir": "blogging",
  "basic_auth": true,
  "username": "username",
  "password": "password",
  "templates": {
    "content_show": "assets/_cms/templates/content_show.jade"
  },
  "css": "assets/css/_blogging.styl"
}
```