Added support to write a non-named list of strings of non-equal lengths

* Added support to write data such as list(c("a", "bb"))

* src/rmatio.c (write_cell): Use ragged parameter when writing a cell

* src/rmatio.c (write_cell_array_with_arrays): Added argument ragged

* inst/tests/test_rmatio.r: More test cases. Removed trailing
  whitespace
Added write_charsxp

* src/rmatio.c: Added write_charsxp

* src/rmatio.c (write_elmt): Added case CHARSXP