add filter for incoming html

adds a htmlparser class that will filter any html but newlines and links
(kept in markdown-like syntax) and converts html-entities to unicode.
add incoming message HTML filter policy

if this policy is on (default), html tags are stripped from incoming
messages. this is useful if your peer uses a graphical client that sends
html formatted messages over otr.

you can turn this off per-peer.
add testcase for IrcHTMLParser