Added persistent version of "start-ssh-agent".

This version keeps the SSH environment variables as non-volatile (persistent).
Useful for "Use Git from the Windows Command Prompt" installation and third party softwares using the "git" command directly such as "[ungit] (https://github.com/FredrikNoren/ungit)".

It should work on Windows XP thought it's not supported anymore.
Feel free to try before officially merging this.

If for some reason you prefer not to integrate it, I've created it as a Gist in case someone like me needs this kind of tool: https://gist.github.com/silverkorn/c86b7607ea22af9ab1f3