#1087 Add folding on square brackets

[Colomban Wendling]

JavaScript (and JSON) has array literals syntax using [ .. ]. Folding these can be convenient when they span several lines.

Attached patch implements this. It doesn't add any new setting and works like { and }. This probably should not change much for languages that only use square brackets for array indexation as the large majority of the time the open and close brackets are placed on the same line in these cases.

[Neil Hodgson]

Committed as [4ccd80].