GitLab

Suggested by Joachim Klein.  Fixes #57.

* src/bin/common_file.hh, src/bin/common_file.cc: New file.
* src/bin/Makefile.am: Build them.
* src/bin/ltlcross.cc: Use the output_file class.
* src/tgbatest/ltlcross3.test: Test >>.
* NEWS: mention it.