Fix useage typo
contextview: Use colors for contexts
contextview: Modify post_filter_command instead to filter items without
a context
Running grep on the colored list would require to catch escape
sequences. This would be a quite ugly task.
Furthermore it is now possible to match for a whitespace before @.
contextview: Don't show the header
context|projectsview: Use postfilter instead of grepping the list to
avoid color escape sequences