Allow empty target

[ysavourel]

In some cases, in XLIFF files, the translator may need to save back an empty target (when the source has some content).

Currently the XLIFF output has the source copied into the target element.

[ysavourel]

Allowed bilingual format to use empty target on output. This resolves issue #10.

→ <<cset 0a2fb9f8575c>>