Allows passing existing keys and salts vs generating afresh

In a multi-server environment, if you want a consistent configuration without having to patch the automatically generated wp-config.php, there should be a way to pass the common keys and salts.
A helper method to prompt the user for some detail
If the `--prompt` arg is specified, prompt the user for arguments
add test for assoc arg with optional value
Exclude assoc args with that have an optional value from being unset

An assoc args with an option value shouldn't trigger the message "--foo parameter needs a value".
Because it doesn't needs a value. Else it would defeat the whole purpose of having `--foo[=<bar>]`.
It needs to be available within the command so you can still do something with it.
Register `--prompt` as a global argument

Doing so is more correct, and avoids this warning:

```
Warning: unknown --prompt parameter
```
Properly set positional arguments
Fix synopsis order

Make it consistent with other core commands.
Merge pull request #699 from wp-cli/fix-synopsis

Fix synopsis order
Switched from --keys-and-salts="..." option to --skip-salts based on @scribu and @danielbachhuber proposal
This should explicitly be a public method.
A functional test for `wp theme path`
Clarification on what this command actually does
Implement `wp theme get`
Use P2 for tests instead, which won't fatal 3.4 (twentyeleven is already installed)
Merge pull request #703 from wp-cli/theme-get-703

Add `wp theme get`
Merge branch 'master' into fix-theme-path

Conflicts:
	features/theme.feature
Use P2, which isn't conditionally present in some WordPress installs
Implement `wp comment get`
Test for `wp comment get`