Expand file_pattern-input to an array

[stefanzweifel]

• refs String values in README.md extended example are now correct #196
• refs pathspec is not respected because of bash expansion #239

[stefanzweifel]

I've tested this change in my test repository here: stefanzweifel/git-auto-commit-action-demo-app#35

The file_pattern works much better now, but there is still an issue with globbing: If an untouched file that matches the pathspec is in the repository, the Action still fails with a Working tree clean. Nothing to commit.-mesage.
However, this can be resolved by adding disable_globbing: true to a workflow.
(Globbing is not disabled by default, due to the fear of breaking the workflow of others. See #154 for details.)

I'm gonna merge this PR to master now. A new version will be tagged after 5-6 days, if no bug reports are coming in.