Add file_glob option to GitHub Releases plugin

Allows you to set the file_glob option to true and then the file option
will be interpreted as a file glob (*, ** and ? wildcards, with \ to
escape them).

This allows deploying files without knowing their exact filenames up
front, for example packages containing the version in their filename.
Merge pull request #181 from dalen/releases_file_globbing

Add file_glob option to GitHub Releases plugin