path/filepath: add example for Ext

path/filepath: add example for Ext

Make it dead simple to see visually what the function outputs in
various scenarios.

Change-Id: I8f6fcd72fa1515361481f0510412cde221e1d4e3
Reviewed-on: https://go-review.googlesource.com/51630
Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Hiroshi Ioka <hirochachacha@gmail.com>