added function to get messages from a channel

[raybuhr]

slackr_channel_history() uses the channels.history endpoint from the Slack web API.

I added some defaults so that the function should work if slackr_setup() had already been ran or if the environment variables were already set. The function parameters should allow the user to have some flexibilty to retrieve messages from various channels at different points in time.

Future extensions could allow user to loop through paginated results instead of retrieving messages only as related to time in Linux format. Could also add option to include if the message had been read in output.

[hrbrmstr]

Lemme review the conflicts and see if I can get this merged this weekend. #ty !

[raybuhr]

Removed the conflicts so this can be auto merged.

[hrbrmstr]

thx!