Feature: prevent cloning of existing repos (all implementations)

[MichaelDimmitt]

Use-case:

We may use repo cloner to clone down repos that may have been missed after a passage of time.
And the repos we already have might be dirty so we cant just delete the folder containing all of them and start again.

As outlined by @kemptosa in his pr: #14

We should do a check when we are about to clone a folder to see if the folder exists before cloning.

[MichaelDimmitt]

All implementations checklist:

• bash
• ruby
• python
• elixir