Allow dash in locale filenames when merging data

[sylvieed]

A project I'm working on uses dashes in locale filenames (i.e. user-en.yml instead of user.en.yml), which was causing i18n-tasks to not be able to write to the files properly (even after editing the write setting in config/i18n-tasks.yml).

This adds - as a valid character before the #{locale} part of filenames.