feat(sourcemaps): add support for pass through of sourcemaps

[admosity]

Adds simple extraction of sourcemaps with https://github.com/webpack-contrib/css-loader delivers #11

Sample config:

{
          test: /\.css$/,
          use: ['to-string-loader?sourceMap', `css-loader?${JSON.stringify({ sourceMap: true, importLoaders: 1 })}`],
        }

Dev console with angular 2 build:

[gajus]

Thank you – is this ready to merge?

[admosity]

Yep it's good to go 👍

[admosity]

Actually.. I retract that statement. I have to rewrite the outputted string in es5.

[admosity]

Okay now it's good.

[OzzieOrca]

I'm not sure this is necessary. The css-loader toString function seems to handle this fine now. I'm using Angular and discovered I could use exports-loader?module.exports.toString(). See #1 (comment).

Angular does currently have an issue where it is stripping sourcemaps from component styles.
I got it working locally by patching my local Angular compiler with angular/angular#16090. It hasn't been merged yet. angular/angular#16088 is related.