Use post_id instead when neither title and post_name are available

Hit this problem when migrating from my wordpress archive. I happened
to have an untitled draft post with both <title> and <wp:post_name>
empty.

Existing migrator will throw the following error:

node_modules/hexo-migrator-wordpress/index.js:50
          postLink = postTitle.toLowerCase().split(' ').join('-');
                               ^
TypeError: Object #<Object> has no method 'toLowerCase'

In this case, post_id is the only thing we can use to avoid conflict,
not pretty but it can only happen to drafts anyway.
WordPress migrator: Trim trailing CRs

Somehow the WordPress export XML from my blog are full of \r\n instead
of \n, but sometimes \n is used as well. It makes the resulting .md
files in neither DOS line endings nor Unix line endings. Converting
all \r\n into \n to make them consistent.
Merge pull request #1 from jjgod/master

Improvements to the WordPress migrator