Add support for multiline description.

README.md

@@ -50,6 +50,8 @@ provides a more detailed description of what the parameters do.
     The first field is the filename of the generated vimdoc help file; the second is the description can you provide
     with this parameter.
 
+    Multi line description can be written using `\n`.
+
 *   **norules**
 
     By default, we generate horizontal rules above level 1-2 headings, as shown below:

vimdoc.go

@@ -373,7 +373,15 @@ func (v *vimDoc) NormalText(out *bytes.Buffer, text []byte) {
 // Header and footer
 func (v *vimDoc) DocumentHeader(out *bytes.Buffer) {
 	if len(v.desc) > 0 {
-		v.writeSplitText(out, []byte(v.filename), []byte(v.desc), " ", 0)
+		parts := strings.Split(v.desc, "\\n")
+		for i, part := range parts {
+			if i == 0 {
+				v.writeSplitText(out, []byte(v.filename), []byte(part), " ", 0)
+			} else {
+				out.WriteString(part)
+			}
+			out.WriteString("\n")
+		}
 	} else {
 		out.WriteString(v.filename)
 		out.WriteString("\n")