No way to manipulate Array or Array elements

[luke-jr]

operator[] returns a const, preventing modifications. Similarly, there is no way to replace items in an Array.

[jgarzik]

2017_modify branch adds erase() support https://github.com/jgarzik/univalue/tree/2017_modify Needs improvement, comments welcome etc.

insert()-at-position should probably also be added. The combination of the two would permit replacing items (and perhaps a two-line inline replace() helper method).