Adding support for bracket notation

[dna113p]

I found this useful myself recently and thought others might as well.

flatten({
     hello: {
       world: {
         again: 'good morning'
       }
     }
   }, {delimiter: '[]' })
//returns:  'hello[world][again]': 'good morning'

unflatten({'hello[world][again]': 'good morning'}, {delimiter: '[]' })
})

//returns:{hello: {world: {again: 'good morning'}}}

[timoxley]

Perhaps mention any two characters can be used. I do worry a bit about the inability to escape any delimited character, though this isn't necessarily specifically a problem for your proposed change.

[timoxley]

What if someone is currently specifying two characters as a delimiter? Perhaps this should be delimiters instead in order to avoid any conflict with that. This then raises question: if someone supplies both delimiter & delimiters? Which is used? both? Perhaps the opt-in signal for this behaviour should then be delimiter + an array (rather than a string).

[FranMackinlay]

This would definitely solve so many issues, good job man! Hope this will get merged soon!