Added slice remapper and improved documentation (#8)

* Experimental slice remapper
* Added iter_slices to DiffOp
* Improvements to the utility diff functions
* Documentation improvements
* More documentation updates on utils
* More documentation on main text diff

examples/original-slices.rs

@@ -0,0 +1,11 @@
+use similar::utils::diff_chars;
+use similar::Algorithm;
+
+fn main() {
+    let old = "1234567890abcdef".to_string();
+    let new = "0123456789Oabzdef".to_string();
+
+    for (change_tag, value) in diff_chars(Algorithm::Myers, &old, &new) {
+        println!("{}{:?}", change_tag, value);
+    }
+}