v.0.6.0

• major internal refactor to simplify how user sessions are handled. We remove the reliance
  on R6. We now just use a regular base R environment to hold the configuration and regular R functions (rather than R6 methods) to handle user sessions. You can now access all polished auth configuration from your Shiny app
  or other R packages that build on polished via the polished::.polished environment.
• new argument "override_user" added to secure_server(). When this argument is set
  to TRUE (the default), the session$userData$user() polished user will be accessible in
  the session$user. Set this argument to FALSE if you are using RStudio Connect or another hosting option that uses the session$user and you need access to the value they set for session$user.
• fix to allow for multiple custom tabItems on the admin panel
• The admin panel query string has been changed from ?page=admin_panel to ?page=admin
• new get_api_key() function that will check for the environment variable "POLISHED_API_KEY"
  if the api key is not found in the polished options.
• new polished_config() function to replace global_sessions_config() which has been
  deprecated.
• Bug Fix (#172) - browser previously refreshed when url query or hash parameters changed, but user remained on the Shiny app. This has been updated to be consistent with normal Shiny behavior (i.e. shiny session does not reload when url query or hash parameters update).
• Added secure_rmd(), which can be used to render and secure any R Markdown (.Rmd) document. Rendering is handled by rmarkdown::render and the then the rendered document is secured with polished authentication.