ENH: a few design ideas

Some ideas I wanted to share:

• Add an ignore marker.
  Currently we have persist (run if output is missing) and skip (skip the task and everything downstream). An ignore marker could skip only the task itself while still allowing downstream tasks to run. This could be useful, for example, when dependencies or products were removed via DVC (or similar tools) and the user wants to ensure the task is not re-run.

• Should the file location be part of the task ID?
  At the moment, a task is re-run if the containing file is moved. An alternative design would ignore such changes if the task content itself did not change.

Review the existing persist and skip marker behavior, then trace how task IDs respond when their containing file moves. Determine the intended semantics and trade-offs for the proposed ignore marker and file-location handling. Done would be an agreed design and documented scope, since the issue names no files or tests.