add globalVars option

This PR adds a `globalVars` option to the middleware, enabling the user to set LESS `@variables` at runtime (based on the application's environment variables for example).

While the solution (prepending a string before parsing) might seem hacky, this seems to be the only way to do this programatically. See the Grunt LESS task doing this the exact same way as a reference: https://github.com/gruntjs/grunt-contrib-less/blob/master/tasks/less.js#L129

This would also close #109