fix defcustom for org2blog/wp-track-posts

be3c8f9b changed this variable from a single string to a list of two
strings, where the second value is the headline to store log entries
under.

Also it appears that the code no longer works if the first value is nil.
Merge pull request #166 from aspiers/fix/wp-track-posts-type

fix defcustom for org2blog/wp-track-posts