modified ggslackr to not require ggplot

This simplified the ggslackr call, relies on built in defaults, but allows user to over ride the defaults through the use of "...".  Also, I removed the dependency on ggplot and now can work with whatever the last plot is (including ggplot objects).
Also, I struggled to get the channel names to work properly, so I also added a function to translate channel names to IDs.  (e.g. "#general" is translated to the C012345678 ID for the channel).