ag: Terminate argument list with '--'

Imported change from the original repository because we override
`ag-argument-list'
ag: Chordless match navigation with 'n' and 'p'.

`ag-mode' is a derivative of `compilation-mode' inheriting error
navigation key map comprised of metakey chords.  Use n/p to navigate
next/previous matches and N/P to jump between matching files.Add "n" and "p" line navigation to `ag-mode-map'