Proper command line parser

[MatheusRich]

Currently, the command line for GoldMiner is bare bones. We can use Ruby's OptionParser to improve that.

Some possible improvements:

• Add a --help/-h option
• Add a --channel option instead of just a plain argument as it's now
• Add a --after option to set the initial date to search messages
• After Automatically create blog post PRs #1, we can add a --dry-run option to just output the blog post, without opening the PR.