Prompt user for input + Optional assoc values (Synopsis)

This PR won't deal with hidden stuff like passwords yet.

First have to deal with that synopsis parser unsets the key so I can't even do a check on it.
New flavour "prompt" by using `( )` instead of the optional `[ ]`. 
Please check the whole `gen_patterns` maybe I am overkilling the aditions.

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"`

**optional asscosiative values**
- [x] Synopsis: optional asscosiative values `--dbpass[=<password>]` and `[--dbpass[=<password>]]`
- [ ] Fix `value-mandatory` `regex`
- [x] Add optional asscosiative values SynopsisParser unit tests
- [ ] Add more SynopisValidator unit tests

**Prompt**
- [x] Implement prompt
- [x] Prompt wp core config
- [ ] Behat wp core config?
- [ ] Prompt wp core install
- [ ] Behat wp core install?
- [x] Fix unit tests
- [ ] More prompts?