Mangen: allow callers to specify header information

We previously had this weak argument called projectName which let you
set a single part of a man page header. Instead do the best we can if
the caller doesn't pass us anything, but let the caller specify anything
they want.

md_docs.go

@@ -108,7 +108,7 @@ func (cmd *Command) GenMarkdownCustom(out *bytes.Buffer, linkHandler func(string
 		fmt.Fprintf(out, "\n")
 	}
 
-	fmt.Fprintf(out, "###### Auto generated by spf13/cobra at %s\n", time.Now().UTC())
+	fmt.Fprintf(out, "###### Auto generated by spf13/cobra on %s\n", time.Now().Format("2-Jan-2006"))
 }
 
 func GenMarkdownTree(cmd *Command, dir string) {