Support filtered handlers

[jwilm]

You generally don't want the bot to interject every time you say ping in a channel. Add a feature where handlers are either unfiltered or filtered by requiring the bot be addressed. For example:

Unfiltered

you: I say ping and the bot responds always
bot: pong

Filtered

you: When I say ping the bot doesn't respond.. until it's directly addressed
# silence
you: bot ping
bot: pong

[jwilm]

Support was added in #20.