Refactoring of argument checking in gitr2 C functions called from R
Improved config listing of variables

* src/git2r_config.c (config): The config variables are now listed by
  level.

* R/config.r (config): Calling config with only the repo argument
  displays the config variables.

* src/git2r_error.h/c: Added error message.
Merge branch 'master' of https://github.com/ropensci/git2r

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Refactoring of config functionality

* The function config has been splitted into two functions. One for
  setting config variables and one for getting.

* src/git2r_config.c/h: Added get_config and set_config. Removed config.

* config.r (config): Changed to call set_config if there are any
  variables to set/delete and get_config to list variables.

* src/git2r.c: Added get_config and set_config and removed config from
  R_CallMethodDef.

* tests/config.R: Fixed test cases