Slug option containing multiple consequent dashes not working as expected

**Describe the bug**

When using multiple dashes in slug template of folder collections like this:

```yaml
slug: "{{year}}_{{month}}_{{day}}_{{hour}}_{{minute}}_{{second}}---{{slug}}"
```

newly created articles have one dash instead of three (as one would hope).

**To Reproduce**

Just use the slug template provided above

**Expected behavior**

I expect slug to be
`2019_01_15_08_20_12---sample-article.md`

but I'm getting

`2019_01_15_08_20_12-sample-article.md`

**Screenshots**

**Applicable Versions:**

- Netlify CMS version: netlify-cms 2.3.3

**Additional context**

This is crucial for me, as I do some string manipulations to generate prettier filenames in Gatsby.