Add a GitHub repo to context

This allows you to ask a question about a GitHub repo:

```
ask --repo egoist/shell-ask "how to use ollama"
```

by default we can limit it to `**/*.md` files, and you can extend it:

```
ask --repo egoist/shell-ask --repo-files "**/*.{md,ts,tsx}" "how does it interact with LLM"
```