Switch off attributes for separators
Append to line only if text has non-zero length
Allow themes to use win.activity. Fix #8

- win.activity can be used to set colors to windows with activity alert
- if win.activity is not defined, alert windows are underscored
- extract win.activity from airline's 'a' section
Use win.activity in iceberg & vim_powerline themes
Disable win activity in powerline and nightly_fox