Update custom importer documentation to advocate returning `null`
Update custom importer documentation to advocate returning `null`

With #1291 custom importers can now return `null` instead of
`sass.NULL`. This means custom importer modules no longer need to
have node-sass as a dependency.

sass/node-sass,nschonni/node-sass,sass/node-sass,xzyfer/node-sass,sass/node-sass,sass/node-sass,Smartbank/node-sass,paulcbetts/node-sass,nschonni/node-sass,paulcbetts/node-sass,ankurp/node-sass,ankurp/node-sass,nschonni/node-sass,saper/node-sass,am11/node-sass,Smartbank/node-sass,saper/node-sass,xzyfer/node-sass,xzyfer/node-sass,am11/node-sass,am11/node-sass,Smartbank/node-sass,sass/node-sass,xzyfer/node-sass,xzyfer/node-sass,paulcbetts/node-sass,paulcbetts/node-sass,Smartbank/node-sass,nschonni/node-sass,ankurp/node-sass,nschonni/node-sass,xzyfer/node-sass,sass/node-sass,saper/node-sass,ankurp/node-sass,nschonni/node-sass,saper/node-sass,Smartbank/node-sass,Smartbank/node-sass,saper/node-sass,saper/node-sass,saper/node-sass,nschonni/node-sass,Smartbank/node-sass,xzyfer/node-sass,sass/node-sass,am11/node-sass