Add sourceRoot option, fixes #7

Hi, I fixed #7 issue.
If no `sourceRoot` option is given, the TypeScript compiler truncates paths in `sourceMap.sources` property.

`sourceRoot` given:
```javascript
{"version":3,"file":"foobar.js","sourceRoot":"./","sources":["src/foobar.ts"],...
```

`sourceRoot` no given:
```javascript
{"version":3,"file":"foobar.js","sourceRoot":"","sources":["foobar.ts"],...
```

So I added `sourceRoot` option to `gulp-type`. Could you review it?
Thanks.