Remove to_s5, to_s, and to_markdown rendering

[bhollis]

to_s5, to_s, and to_markdown are all not particularly useful, and should be removed or moved into plugins.

[distler]

to_s and to_markdown are not even implement (at least, not for real).

to_s5, on the other hand, is both fully implemented and very useful.

[bhollis]

If s5 is useful, it should be moved into a plugin.