Add default configuration for meld diff tool.

This adds the command line and typical location for the meld.exe diff tool.

An all-in-one package for meld on windows can be found here : https://code.google.com/p/meld-installer/
(Unfortunately it has a bug related to relative paths, and fails to find some of the files passed by git difftool. A small modification can be applied as underlined in this bug report : https://code.google.com/p/meld-installer/issues/detail?can=1&start=0&num=100&q=&colspec=ID%20Type%20Status%20Priority%20Milestone%20Owner%20Summary&groupby=&sort=&id=11)