Add block help and source for external tab

[cyderize]

Adds block help and source for the external tab blocks, leaving all blocks documented. Closes #330.

Blocks documented:

• load graph from url
• friends of
• loved tracks of
• hypernyms of
• hyponyms of
• synsets of
• definition of
• movie #s where title has
• person #s where name has
• title of movie
• cast of movie
• movies with person
• name of person

[stevenbird]

Thanks @cyderize, this is excellent.