`gg clone` should ignore if the target directory already exists. Can be overriden via a force flag.

[jpninanjohn]

gg clone now always clones all the specified remotes in gg conf and those passed in args. If the target directory already exists, it should not clone again. It should ignore unless a force flag is given. Force flag should also be in the conf for each entry defaulting to false.

This will avoid overwrites when calling gg clone with the same conf file.