factorize component frontmatter defaults

[sebastienbarre]

Hey folks,
A quick improvement to the build options, trying to address two issues:

• the options that can be overridden in the component front matter are not documented,
• the default values for these options are buried inside the templates.

I moved all of them with the rest inside src/build-components-defaults:

• it is now easier to change these defaults (you can get rid of all your "bodyClass" : "bg-white" for example),
• they are documented,.
• you can override them from build.js, the same way you can override the other options,
• all the defaults have been organized into sub-objects, for clarity.

[sebastienbarre]

@mrmrs I also figured out a way to find when a component was first introduced to the repo, using nodegit, and this should be fairly resistant to you moving/renaming components around. This will have to be in another PR, but I will whip out a quick, alternate index page to demonstrate, where components will be ordered by date, most recent first.