Support unstack from quickfix

[dsirov]

If the quickfix window is (somehow) populated with execution output and contains stack traces, unstack does not identify it (I guess because of prefix '||')

|| [python ./test.py]
|| Traceback (most recent call last):
||   File "/tmp/test.py", line 10, in <module>
||     main()
||   File "/tmp/test.py", line 7, in main
||     test('1')
||   File "/tmp/test.py", line 4, in test
||     print( s + str( 1 / 0 ) )
|| ZeroDivisionError: division by zero
|| [Finished in 0 seconds with code 1]