History and duration predictions

[lexman]

For the moment, tuttle only remembers the logs for the last run.

An option (in the command line ?) would allow preserving the full history.

From this history, projection of execution of each process (and the whole workflow) could be computed, like in Jenkins.

A tuttle invalidate all command could reset the whole process without removing history (better than removing the .tuttle directory

Implementation would be easyier if workflow's state is in a database. Eg a sqlite database