Handle GitHub usernames case-insensitively

[necojackarc]

As we found out in #31, you need to be careful about case sensitivity when you configure this GitHub Action.

Since GitHub usually handles usernames case-insensitively, it's more convenient to handle usernames in this action case-insensitively. What we need to do is probably:

• Lowercase usernames and group names in the config file
• Lowercase the author name

Once we do these things before any comparisons, we can handle them in a case-insensitive way.