Fixes to Basque translation.
Fix --add-comments breakage in default parser configs.

Commit 025eccd12e1167ab42ae6db4000290e7d6d494e8 incorrectly omitted
space before --add-comments flag.  This resulted in broken scanning of
sources for anybody who installed a new install of 1.5.6 (as opposed to
upgrading from previous versions).

Correct this and also include code to automatically fix borked
configurations.

Fixes #536.
Increase version to 1.5.7, update NEWS.
Merge branch 'stable'