Slackr 3.3.0

[mrkaye97]

Closes #188 #187

User-facing changes in this version:

• ggslackr now relies on dots (...) to pass arguments through to ggsave
• ggplot2 is now in suggests, so the user doesn't need it installed to use slackr. ggslackr will prompt to install if it's not already.
• Fixes a "bug" with pagination in slackr_history causing an almost infinite loop
• Fixes a bug in slackr_save where initial_comment would do nothing
• Improves documentation of slackr_history to be more helpful in pointing the user to the Slack API docs

Backend changes

• Gets rid of default args in some un-exported functions to make tracking down bugs easier
• Adds significant test coverage