fix improper carrot escaping, fixes #674

re-introduced the `:filter_html` option. This means behaviour such as: 

`2 <5    but you need to see this 3> 4   not` 
=> 2 <5    but you need to see this 3> 4   not

`2 < 5 but you need to see this 3 > 4 not` 
=> 2 < 5 but you need to see this 3 > 4 not

---

i think this is the best solution for the moment as it will bring back all < markdown quote functionality and &&'s in code blocks behave (as html_escape is no longer necessary when filter_html is active).

Downside(s):

- if a user is tech enough to know tags like `<a href='bobo'>here</>` the output will be `` ... but they'll probably then be smart enough to realise what's happened/ someone will point out their comment makes no sense.
- if a user accidentally puts carrots directly next to text their text will be *nuked*. again they will probably see this .... and it's an edge case which probably involves mathematicians and emoticon fans...