use console logger

[romainfrancois]

closes #48

This uses a xeus::make_console_logger(xeus::xlogger::full) for now, perhaps later we should use a xeus::make_file_logger as well, perhaps based on an R option, e.g. in IRkernel :

#' Kernel logging functions
#'
#' A set of exported logging utilities that have the capability to be used in upstream projects.
#' Log level and log file can be set via R package options e.g. \code{options(jupyter.log_level = 2L)}
#' or from the environment variables JUPYTER_LOG_LEVEL and JUPYTER_LOGFILE.

[romainfrancois]

I'm not sure how to send a log message to the kernel's p_logger directly, so that e.g. the R code might contribute to the logs.

[romainfrancois]

I'll follow up in a different issue/pr.