Feature: Add ability to parse string of maxAge
It is difficult to set maxAge to milisecond for large value such as month or year.

It would be great to be able to do something like:

```
const CONFIG = {
  maxAge: '1 month 2 weeks'
}
```
instead of
```
const CONFIG = {
  maxAge: 3839343830 // yucks
}
```

If this is a good one to have, I can make a PR for it. (as implemented in my [next-session](https://github.com/hoangvvo/next-session#options), see the bottom of the table)

