Add support for qs options via qsOptions key

[simov]

Closes #1473

The qsOptions key can be used to pass options to both the parse and the stringify methods of the qs and querystring modules.

The arrayFormat options was missing from the qs readme ljharb/qs#72

[nylen]

A few thoughts on this:

1. Can we link directly to the relevant options from the readme?
   • qs.parse
   • qs.stringify
   • querystring.parse (seems better to link directly to a specific Node.js version here, rather than the latest)
   • querystring.stringify
2. Not our issue, but it's hard to tell from a glance what options the qs methods support.
3. Since this does a lot of different things, may be better to split it into two: qsParseOptions and qsStringifyOptions?
4. Some text like this might be good, too:

qsOptions - object containing options to pass to the qs or querystring module_, for example, to change the way arrays are converted to query strings_

[simov]

Agreed, I was thinking about the same problem - different options for parse and stringify Also adding links there would be good.

[simov]

Improved the docs a bit, and introduced the qsParseOptions and the qsStringifyOptions keys.