testium v0.4

• Start/Pause and Stop moved to the step toolbar, which is now always
  visible. F5 starts, pauses or resumes; Shift+F5 stops.
• The step toolbar shows large icons when docked at the top, small ones
  elsewhere.
• Fix: the green "current item" line stayed in the test tree after
  stopping a paused test.
• Custom report exporters (testium.exporters pip plugins) now work
  in every install channel. Install them with the host Python — the
  python_bin interpreter, as for py_func dependencies.
• New command report export: post-process the report with any
  external tool — cmd: mytool $(db) $(out) receives a SQLite copy
  of the report and the output path.
• New testium_report helper for writing exporters: header, typed
  rows, filters and tree without SQL. Nothing to install for plugins;
  pip install testium provides it in a development venv.
• New tutorial: doc/exporter_tutorial.md.