Added functionality to delete a config entry

* R/config.r (config): Setting a config variable to NULL deletes the
  config variable from the config file with the highest level (usually
  the local one).

* src/git2r_config.c (config): Fixed C code to delete the config entry
  if the variable is NULL.

* man/config-methods.Rd: Updated documentation

* tests/config.R: Added test case