Remove mh-pack.c++

This is no longer needed and just got left in there.
Add some "mhng-pipe" programs

The general idea behind these sorts of programs is that I want to have
MH interfaces that are easy to parse from a shell and are guarnteed
not to have any side-effects.
Add a preliminary bash completion file

This currently isn't particularly useful because I can only
tab-complete sequence numbers.  This isn't useful because sequence
numbers tend to be at most 2 characters long, which makes the tab
pretty useless.