Implement setup script

[barryhughes]

The readme lists a number of areas in which a search and replace should be performed. I think it could be handy to have a script that guides users through this:

• Could be a standalone PHP script that can be executed using something like ./bin/setup (ie, it would not depend on WP CLI nor would it be a Composer script)
• It could validate that the plugin slug (for example) is a valid slug before performing search-and-replace
• Could provide a final summary of the changes and a chance to bail out or make changes
• It could run composer install at the end

Conceptually, what do you think—worthwhile/maybe useful but not sure/meh?

[cliffordp]

Yes, sir! Please see #73

[barryhughes]

Hah! I promise I looked for an existing issue first of all ;-)