This patch implements I_S.PLUGINS.LOAD_OPTION column
as specified by WL#5496.

mysql-test/r/plugin_load_option.result:
  A test case for WL#5496.
mysql-test/suite/funcs_1/r/is_columns_is.result:
  Adjusted a test case according to WL#5496.
mysql-test/t/plugin_load_option.test:
  A test case for WL#5496.
sql/sql_show.cc:
  Added LOAD_OPTION column to I_S.PLUGINS.