Add --export=<file> option to search-replace (opt-out of in-place replacements)

When the `--export` option is provided, search-replace then behaves similarly to db-export. Instead of doing the search-and-replace on the database in-place, the --export option will generate all of the `DROP TABLE`, `CREATE TABLE`, and `INSERT` statements to load the data on another server.

Doing a `wp search-replace --export=<file>` seems to be two times faster than `wp search-replace`, since it is not doing any `UPDATE` statements.

Fixes #709