i18n: Make .underscore files translatable

Currently the strings in .underscore files are not extracted
by i18n/extract.py, which causes most strings in such files
are not translatable. As there is no existing tools which can
extract messages from these files, a simple shell command is
written to do such job.
Merge pull request #3704 from louyihua/localization-fix

i18n: Make .underscore files translatable