Add proxy support for extension installation.

Configurable via a "proxy" preference (which could be used for other parts of
Brackets that should use HTTP proxy).

This fixes #6241, #5958 and [this backlog item](https://trello.com/c/SEV95fdK/902-support-for-a-proxy-in-extensions-installation)
Merge pull request #6911 from adobe/dangoor/extman-proxy

Add proxy support for extension installation.