ColTypeDate: Add relative formatter

ChangeLog

@@ -40,6 +40,7 @@
 - Removed unused 'dom' and 'shell.prompt' configuration settings.
 - Numerous performance improvements. Taskwarrior 2.5.1 is between X% and
   Y% faster than 2.5.0 when running various commands.
+- New formatting specifier 'relative' for columns of Date type was introduced.
 
 ------ current release ---------------------------
 

src/columns/ColTypeDate.cpp

@@ -45,6 +45,7 @@ ColumnTypeDate::ColumnTypeDate ()
                 "epoch", 
                 "iso",
                 "age",
+                "relative",
                 "remaining",
                 "countdown"};
 
@@ -55,6 +56,7 @@ ColumnTypeDate::ColumnTypeDate ()
                now.toEpochString (),
                now.toISO (),
                ISO8601p (ISO8601d () - now).formatVague (),
+               "-" + ISO8601p (ISO8601d () - now).formatVague (),
                "",
                ISO8601p (ISO8601d () - now).format ()};
 }
@@ -115,6 +117,14 @@ void ColumnTypeDate::measure (Task& task, unsigned int& minimum, unsigned int& m
       else
         minimum = maximum = ISO8601p (date - now).formatVague ().length () + 1;
     }
+    else if (_style == "relative")
+    {
+      ISO8601d now;
+      if (now < date)
+        minimum = maximum = ISO8601p (date - now).formatVague ().length ();
+      else
+        minimum = maximum = ISO8601p (now - date).formatVague ().length () + 1;
+    }
     else if (_style == "remaining")
     {
       ISO8601d now;
@@ -177,6 +187,14 @@ void ColumnTypeDate::render (
       else
         renderStringLeft (lines, width, color, "-" + ISO8601p (date - now).formatVague ());
     }
+    else if (_style == "relative")
+    {
+      ISO8601d now;
+      if (now < date)
+        renderStringLeft (lines, width, color, ISO8601p (date - now).formatVague ());
+      else
+        renderStringLeft (lines, width, color, "-" + ISO8601p (now - date).formatVague ());
+    }
 
     else if (_style == "remaining")
     {