Add new sort option .name and .Name

Add two new sort options `.name` and `.Name` which with ignore a leading
`.` if present on the file name before sorting according to `name` and
`Name`.

This new sort is convenient if you want to list hidden and unhidden
files sorted together.

src/fs/filter.rs

@@ -191,6 +191,10 @@ pub enum SortField {
     /// bad, even though that’s kind of nonsensical. So it’s its own variant
     /// that can be reversed like usual.
     ModifiedAge,
+
+    /// The file's name, however if the name of the file begins with `.`
+    /// ignore the leading `.` and then sort as Name
+    NameMixHidden(SortCase),
 }
 
 /// Whether a field should be sorted case-sensitively or case-insensitively.
@@ -253,6 +257,23 @@ impl SortField {
                 Ordering::Equal  => natord::compare_ignore_case(&*a.name, &*b.name),
                 order            => order,
             },
+
+            SortField::NameMixHidden(ABCabc) => natord::compare(
+                SortField::strip_dot(&a.name),
+                SortField::strip_dot(&b.name)
+            ),
+            SortField::NameMixHidden(AaBbCc) => natord::compare_ignore_case(
+                SortField::strip_dot(&a.name),
+                SortField::strip_dot(&b.name)
+            )
+        }
+    }
+
+    fn strip_dot(n: &str) -> &str {
+        if n.starts_with(".") {
+            &n[1..]
+        } else {
+            n
         }
     }
 }

src/options/filter.rs

@@ -41,6 +41,12 @@ impl SortField {
         else if word == "Name" || word == "Filename" {
             Ok(SortField::Name(SortCase::ABCabc))
         }
+        else if word == ".name" || word == ".filename" {
+            Ok(SortField::NameMixHidden(SortCase::AaBbCc))
+        }
+        else if word == ".Name" || word == ".Filename" {
+            Ok(SortField::NameMixHidden(SortCase::ABCabc))
+        }
         else if word == "size" || word == "filesize" {
             Ok(SortField::Size)
         }
@@ -231,6 +237,9 @@ mod test {
         test!(newest:        SortField <- ["--sort=oldest"];   Both => Ok(SortField::ModifiedAge));
         test!(age:           SortField <- ["-sage"];           Both => Ok(SortField::ModifiedAge));
 
+        test!(mix_hidden_lowercase:     SortField <- ["--sort", ".name"];  Both => Ok(SortField::NameMixHidden(SortCase::AaBbCc)));
+        test!(mix_hidden_uppercase:     SortField <- ["--sort", ".Name"];  Both => Ok(SortField::NameMixHidden(SortCase::ABCabc)));
+
         // Errors
         test!(error:         SortField <- ["--sort=colour"];   Both => Err(Misfire::BadArgument(&flags::SORT, OsString::from("colour"))));