fix(editline): firstline has different column calculation (#132)

It should take a possible prefix into account.

Author: Tieske

spec/09-editline-format_spec.lua

@@ -980,7 +980,7 @@ describe("EditLine:", function()
               'some test data|'
             },
             1,
-            15,
+            55,
           }, {testwrap({
             width = 60,
             first_width = 20,

src/terminal/editline.lua

@@ -612,7 +612,7 @@ do
         if l < first_width then
           -- cursor is on a first-line, on a regular position
           cur_line = 1
-          cur_col = l + 1
+          cur_col = l + 1 + (width - first_width)
         end
       else
         local l = lines[#lines]:len_col()