Support using OpenSSH as well as plink

A few people have had a bit of a setup task to get Pageant configured with their SSH keys, because they've followed instructions from GitHub or SmartGit in the past which uses the Cygwin port of OpenSSH instead.

While you can import OpenSSH keys into PuTTY format using PuTTYgen, it is an additional step, and it would be nicer to just work with this existing setup.

Ideally we could have a setting in Preferences to choose between Plink and SSH, and automatically detect which one to default to depending on the users current setup.