feat: enable to use multi dirs for each workspace

[delphinus]

Fix #264

Now you can set multiple directories into each “workspace”.

workspaces = {
  CONF = "/home/my_username/.config",
  DATA = "/home/my_username/.local/share",
  WIKI = "/home/my_username/wiki",
  PJT = {
    "/home/my_username/project_foo",
    "/home/my_username/project_bar",
    "/home/my_username/project_foobar",
  },
}

If you call :Telescope frecency workspace=CONF, you can get entries filtered out with the directory (~/.config) itself. In case :Telescope frecency workspace=PJT, you can get combined results with entries under each directories (~/project_foo, ~/project_bar and ~/project_foobar).