Added 'basePath' param for compilation

Related to [@1460](https://github.com/google/traceur-compiler/issues/1460)

Added 'basePath' param for compilation. If not defined - will work as work before. Work for @import and System.map.

```js
var Traceur = require('traceur');

var dist = '...', // add dist dir

// Fill System.map
System.map["foo"] = "bar/baz/foo";

var options = new Traceur.util.CommandOptions(),
  sources = [
    {
      name: 'boot.js', // replace with yours
      type: 'module'
    }
  ];


options.basePath = './src/application/es6/';


Traceur.recursiveModuleCompileToSingleFile(dist, sources, options).then(function() {
  process.exit(0);
}).catch(function(err) {
  var errors = err.errors || [err];
  errors.forEach(function(err) {
    console.error(err.stack || err);
  });
  process.exit(1);
});
```