ensurePrefix 
Ensure that a prefix exists in the string, and add it if it does not exist
Usage 
ts
import { ensurePrefix } from 'rattail'
ensurePrefix('prefix-hello-world', 'prefix-') // return 'prefix-hello-world'
ensurePrefix('hello-world', 'prefix-') // return 'prefix-hello-world'Arguments 
| Arg | Type | Defaults | 
|---|---|---|
value | string | |
prefix | string | 
Return 
| Type | 
|---|
string |