Change skipexists to stopifexists

We have a option in the client class that when set will skip existing
repositories, and which is set by default. We want to add a command line
flag that allows the user to enable this behavior. What would this flag
be called? Probably something like --noskipexists. This is not really a
good name since it doesn't say what the problem will do, only what it
won't do; and there's no really good alternatives.

We change the name of the option from skipexists to the more descriptive
stopifexists. It says what the program will do, and allows us to simply
add a command line flag later on with the same name.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Add --stopifexists flag

We have an option in the Client class that when set will make the
program stop upon attempting to clone a repository that already exists.
It is not set by default, and we want the user to be able to enable this
behavior.

Add a command line flag for the stopifexists option.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>