Trailing slash at the beginning should be automatically added if not present

[fr3fou]

Running vercel-redirects foo.com bar will result in a redirect that can't be accessed:

{
	"redirects": [
		{ "source": "bar", "dest": "foo.com" }
	]
}

An extra / should be added before bar, like so

{
	"redirects": [
		{ "source": "/bar", "dest": "foo.com" }
	              // ^ right here
	]
}