Optimize Git handling

[rhuss]

If external blocks are used fish-pepper always clone or pulls a github repository. This could be optimized by

• introducing an update interval so that git operations only happen e.g. once a day
• add command line options like --oflline (offline) --update (update external github refs)
• Check whether a tag is used and whether the HEAD of the currently checked out repo points to this tag. If so, resign from doing an update.
• Don't know whether this is possible, but if one could check cheaply whether the remote repo has been changed then one could use this check, too to decided whether to pull or not.