Fix custom flag not behaving correctly.

Instead of prefixing or appending the option flags, this allows the user
to specify the full option flag between the extract command and
filename. So instead of the default 'xf' flag, the user should specify
'xvzf' instead of trying to guess whether to add the 'vz' flags.
Merge pull request #20 from nanliu/tarflag

Fix custom flag not behaving correctly.