fix wrong sourceMapUrl

fix #100

In this case ( as a middleware ), `sourceMapBasepath` doesn't need to be customized by users, we just generate the right path for each request automatically. So the options exposed can just stay the same.

Two things need to be noticed about this request, and are mainly about working with chrome devtool, so that we can achieve in-browser editing less alone with live reloading style and changing your local less files directly:

1. use separate source-map file instead of inline-sourcemap with css file. 

2. handle `*.css.map` —— source-map request. Since the mechanism that chrome devtool use to reload css style whenever .less file changes is to refetch the source-map file, so it is necessary to handle source-map request to ensure chrome devtool get the update, otherwise, the style will not be reload though .less file has changed locally.
Instead of adding the extra check in the `isValidPath` how about updating this regex? `/\.css(\.map)?$/`