allcontributors[bot] mentions itself as missing contributor

- `all-contributors-cli` version: 6.15.0
- `node` version: v12.16.1
- `npm` (or `yarn`) version: 6.14.0

**Relevant code or config**

```bash
➜ npx all-contributors-cli check
```

**What you did:**

1. I made the `all-contributors` bot open a few pull-requests to get people added to the readme. (by mentioning the bot in a couple of issues)
2. If I now run the `all-contributors-cli check` script at my terminal, to see if I've missed someone, it mentions that the bot itself is missing.

**What happened:**

```bash
npx: installed 109 in 4.949s
Missing contributors in .all-contributorsrc:
allcontributors[bot]
```

**Reproduction repository:**

```bash
git clone git@github.com:smeijer/testing-playground.git
cd testing-playground
git reset 7a66910ee69423151e222a308cbe66a945d961bc --hard

npx all-contributors-cli check
```

**Problem description:**

I realize, this project is about acknowledging ALL contributors, but still. Even the bots?

**Suggested solution:**

Add an option to ignore bots. (or just make it the default)