This CL implements a new godoc feature to save the search
index on disk. Use -write_index to create the search
index file named with -index_files. Use -index_files to
provide a glob pattern specifying index file(s) when
starting godoc; in this case the run-time indexer is not
run.

Known issues:
- saving/restoring full text index is not yet supported
- the list of flags and overall usage logic could use a
  cleanup

R=rsc, dsymonds
CC=golang-dev
https://golang.org/cl/4974045