v0.6.8

Feature:

• New action: if-query-empty, if-query-not-empty. Execute actions on
  certain query conditions.
• New action: append-and-select allows you to append current query to the
  item pool and select it. It would help to turn skim into a tag manager
  where new tags could be added to the candidate list.

Bug fixes:

• Fix #188: crates.io breaks on 0.6.7
• Fix: run_with will break if called multiple times from the same process.
• Update nix to 0.14