Support the indented syntax

[roryokane]

There should be a syntax selector on the page that lets you choose between the default (SCSS) and indented (Sass) syntaxes.

The selector control could be

• a drop-down menu from clicking the "Sass" label in the top left (make the label "Sass ▾")
• a menu or radio buttons directly to the right of the label
• a menu or radio buttons aligned to the right border of the Sass textarea
• radio buttons in the footer of the page.

It looks like right now, the SCSS syntax is hard-coded in app/controllers/main_controller.rb.