Add clipboard support on Cygwin with putclip

* Add putclip from cygutils-extra as a command for clipboard_open and
  mention it in the docs.
* Add lpass.exe to .gitignore.

Signed-off-by: Douglas Thrift <douglas@douglasthrift.net>
Merge pull request #30 from douglaswth/cygwin_putclip

Add clipboard support on Cygwin with putclip