Do not remove quotation marks from frontmatter

**Describe the bug**

I have a value like this in my frontmatter:

```
coords:
- "1,4,5,3,45,34"
```

However, after editing the post the quotation marks are removed so, and now it is no longer considered a string in liquid. That means I can no longer just {{ c }}, as this will output `14534534` and not like `1,4,5,3,45,34` which it would before removing the quotation marks.

**To Reproduce**

The quotation marks seems to always be removed like here - https://github.com/motionsplan/motionsplan-dk-jekyll/pull/74/files#diff-e2e171531dbff3ff531fa7df1eb53dc5720cd96416379996ffd3e7306bbd73f2L36

**Expected behavior**

Make it possible to have a comma separated list as a string. Maybe a setting on the individual widget.

**Screenshots**

**Applicable Versions:**

- Netlify CMS version: 2.10.131
- Git provider: [e.g. GitHub, BitBucket]
- OS: [e.g. Windows 7]
- Browser version [e.g. chrome 22, safari 11]

- Node.JS version:

**CMS configuration**

https://github.com/motionsplan/motionsplan-dk-jekyll/blob/master/admin/config.yml

**Additional context**