Remove import_plugin method
Scan Sass input on Gist save for @imports and add them to the frontmatter
Add Normalize SCSS and bump all gems
Revert "Remove import_plugin method"

This reverts commit 0afb0dd89208198c0fdc5c56ac851059b94ce46f.
Refactor method of parsing Sass input for imports
Refactored gem loading, going back to requiring "on-demand"

Found a conflict between normalize-scss and responsive-sass.
Responsive-sass includes a file called 'normalize' which can be
optionally @imported. So when responsive-sass was required at runtime,
@import "normalize" was importing the file from responsive-sass, not
normalize-scss.
Remove unnecessary unpack
Tweak frontmatter format
Tweak require_plugins
A significant rewrite of the frontmatter parsing algorithm