Break loaders/wrappers into plugins

[KyleAMathews]

There are potentially dozens of different file types people would want to use with Gatsby + many different ways to prepare Markdown and other file formats. To simplify (code + conceptually) adding or modifying how files are loaded/wrapped, we should create a plugin architecture + break out existing markdown/html loaders/wrappers into plugins that are installed separately from Gatsby core.

[KyleAMathews]

/cc @davad

[KyleAMathews]

This is more-or-less fixed as of https://github.com/gatsbyjs/gatsby/releases/tag/v0.7.1