Configurable content directory

[tmcw]

This is currently hardcoded to _posts. I fear the complexity of the Full Solution to this problem but it looks like Hugo etc are being special, so allowing custom globs is probably necessary.

[tmcw]

I think this will require actions/core to read an option set in the workflow YAML.

[mattdsteele]

I have a need for this for my Gatsby-powered blog - I'll submit a PR when it's ready!