Allow YAML string fields to be explicitly quoted when serialising front matter

I have run into an interoperability issue when a string field contains a value such as a 24-hour time.

For example, if a collection contains:
```
- label: "Start Time"
name: "start_time"
widget: "string"
```

If I enter: `18:30`, decap serialises it as `start_time: 18:30`.

I think Decap correctly treats this as a string. However, Jekyll’s YAML parsing interprets the unquoted value 18:30 using YAML 1.1 sexagesimal notation, resulting in: `1110`.

This means the value is rendered by Jekyll as 1110/1110.0 rather than 18:30.

Quoting the value fixes the issue:
`start_time: "18:30"`

## Expected behaviour
Given: `18:30`, Decap would serialise this to: `start_time: "18:30"`.

## Current behaviour
Decap serialises: `start_time: 18:30`