Addition of '--file' option to 'bst artifact log'

Summary

[//]: It could be nice to be able to write the output of 'bst artifact log' to a file if wanted, as this makes it easier to save the log if one may wish to view it at a later point.

mentioned in merge request !1552 (merged)

closed via merge request !1552 (merged)

mentioned in commit 8adb55af