Allow arbitrary command (via child process?)

I currently use wait-on for http but have to write something separate for other cases.

E.g. because mysql listens on it's port even when it's not ready to accept connections, i do something like this:
```shell
#!/usr/bin/env bash

set -e
i=0
while ! mysqladmin ping -h"$DB_HOST" --silent; do
i=$((i+1))
if [[ "$i" -gt '20' ]]; then
echo "mysql failed to start"
exit 1
fi
sleep 1
done

npx wait-on http://somehttpcheck
```
But if wait-on supported a `command` resource i could do this:

```shell
npx wait-on http://somehttpcheck "command:mysqladmin ping -h$DB_HOST"
```
I'm not sure `command` is the right naming but hopefully the intention is clear.