bem-xjst: support previous source maps

[miripiruni]

fix for #283

cc @sbmaxx

Related:

• https://github.com/enb/enb-source-map/pull/19
• https://github.com/bem/bem-xjst/pull/491

[miripiruni]

@tadatuta @zxqfox review needed

[tadatuta]

Please update README according to changes

[qfox]

Great!

[miripiruni]

• enb-bemxjst@8.9.0