Add filter to post_meta value

[sboisvert]

In some cases it is useful to have a more granular control over what post_meta is shown to the end user. Or modify some values before showing them to the user (json_decode, base64_decode, etc)

[trepmal]

Note: this PR addresses also #7 and #18 (unserializing data). I see no harm, but we may want to reconfirm after all these years.