cmd/present: fix displaying line numbers for .code and .play
cmd/present: fix displaying line numbers for .code and .play

Line numbers match line numbers from the included file,
not the lines in the segment selected by the
.code or .play START/STOP regexps.

Fixes golang/go#5248

Change-Id: I374e7b665e98db5032446ae49e6fae00d9d2b975
Reviewed-on: https://go-review.googlesource.com/20311
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/tools,syreclabs/go-tools,golang/tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools