Filter syndication targets by post-type, specify checked as appropriate

[barryf]

Support filtering of syndication targets using a post-type parameter so that only the appropriate targets are returned for the chosen post type. For example, for bookmarks, select a bookmarking provider.

Also, support a checked property for when the user is likely to always want the target selected. For example, if the user always wants notes syndicated to Twitter.

• Filter by post-type Additional syndicate-to property indieweb/micropub-extensions#23 (comment)
• Checked property: Additional syndicate-to property indieweb/micropub-extensions#23