Delete local cache by default

[mnuttall]

After one call to promote, it is necessary to rm -rf ~/.promotion/cache before the next. Modify the CLI so that the cache is deleted by default. Add a --keep flag to prevent cache deletion.

[bigkevmcd]

Add a Remove() error method to the Repository and trigger it (defer) at the end of the Promote method.

Inside Remove(), use https://golang.org/pkg/os/#RemoveAll to delete the tree.

[dibbles]

I'll take a look at doing this.

[dibbles]

This can be closed by someone with relevant powers. @mnuttall @bigkevmcd