Add Cygwin pipe hack.

The ! command is useful for filtering things through programs, but since
I'm usually using Windows, I don't have all the useful Unix
text-processing programs... except through Cygwin. So I added a mapping
for filtering things through programs in the cygwin bin directory.
Add function for creating /cygdrive/ paths.

Turn paths like this
C:\Users\Pandu\Dropbox\
Into paths like this
/cygdrive/c/Users/Pandu/Dropbox/
Not quite complete -- doesn't work for paths containing parens and other
weird things