Prompt user for input - round 2

This PR won't deal with hidden stuff like passwords yet.

A. No password, no prompt
`wp core config --dbname=foo --dbuser=ano`
B. Password flag, prompt
`wp core config --dbname=foo --dbuser=ano --dbpass`
C. Password assoc arg, no prompt
`wp core config --dbname=foo --dbuser=ano --dbpass="secret"`

**Prompt**
- [x] Implement prompt
- [x] Prompt wp core config
- [ ] Write tests