Add option to hide label text for labeled points

[jemissik]

Users would like the option to hide the label text for the labeled points, so that they can just show the markers.

This could be fixed at the same time as #98

[jemissik]

It should probably also still work if the label field is blank

[jemissik]

As a workaround, you should be able to get blank labels by entering triple quotes in the label field, like this:

longitude,latitude,label
-123,56,""""""
-121.5,54.5,""""""

If you edit the csv in excel and enter "" in the label column, it should save like this.

I'll fix it so that it still works if that field is left blank.