Add a way to convert svn externals

[FearlessHyena]

There seem to be certain ways to do this http://patrickbougie.com/2013/03/18/convert-svn-to-git-repository/ http://liyanage.github.io/git-tools/

It would be a great addition to this tool

This can probably be done using a local directory as the source rather than the online svn repository and adding the external as a submodule as mentioned in the first link above

git submodule add git://github.com/pbougie/<repository>.git path/to/<repository>

There might however be a better way to do it