[Feature request] Channel exempt from mapping/filters

[Xtrah]

Would be nice to have a possibility to add channels to a list which makes the bot ignore them. This gives users the option to create and join certain public channels freely, for example for a game that some people plays but others don't.

Right now the Slackbot forces every mapped user to be a member of channels specified in SeAT and there are no freedom to join/leave open (public) channels which makes the bot less flexible.