Add multiline support to `wp shell` via backslash

We could remove the -r flag from the `read` builtin, but that would have
several drawbacks:

* couldn't dynamically change the prompt
* would add another layer of escaping, on top of PHP's eval()
update shell man page

[ci skip]
Merge pull request #432 from wp-cli/shell-multiline

Add multiline support to `wp shell` via backslash