Add comma format to the arrayFormat option

[zepod]

This pull requests adds fourth arrayFormat option, named comma. We needed this for our project, as we needed to support arrays in query in below shape...

foo=1,2,3,4,5,6,7,8

This format is mainly used to shorten URLs which include things as identifiers, where there is expected to be a lot of them.

On the way to add this feature I had to slightly reimplement encoderForArrayFormat to use Array.prototype.reduce instead of previous for .. in

[tomaass]

Great work @zepod !
Fix tests please I really want to use it on my project 🥇

[sindresorhus]

Why did you have to switch to Array#reduce?

[sindresorhus]

You have to include TypeScript types for the changes too, since a TypeScript declaration file was recently added to master.

[zepod]

@sindresorhus
the way it was implemented with for encoderForArrayFormat was a unary function mapping single element of provided array, to single key in the query sequence.

Using comma format, we wanted to map the multiple elements of array to a single key in the query.

.. Typescript types I will deliver

[zepod]

Typescript types added

[sindresorhus]

Looks good. Thank you :)