Added sort keys operator

pkg/yqlib/doc/Sort Keys.md

@@ -0,0 +1,68 @@
+The Sort Keys operator sorts maps by their keys (based on their string value). This operator does not do anything to arrays or scalars (so you can easily recursively apply it to all maps).
+
+Sort is particularly useful for diffing two different yaml documents:
+
+```bash
+yq eval -i 'sortKeys(..)' file1.yml
+yq eval -i 'sortKeys(..)' file2.yml
+diff file1.yml file2.yml
+```
+
+## Sort keys of map
+Given a sample.yml file of:
+```yaml
+c: frog
+a: blah
+b: bing
+```
+then
+```bash
+yq eval 'sortKeys(.)' sample.yml
+```
+will output
+```yaml
+a: blah
+b: bing
+c: frog
+```
+
+## Sort keys recursively
+Note the array elements are left unsorted, but maps inside arrays are sorted
+
+Given a sample.yml file of:
+```yaml
+bParent:
+  c: dog
+  array:
+    - 3
+    - 1
+    - 2
+aParent:
+  z: donkey
+  x:
+    - c: yum
+      b: delish
+    - b: ew
+      a: apple
+```
+then
+```bash
+yq eval 'sortKeys(..)' sample.yml
+```
+will output
+```yaml
+aParent:
+  x:
+    - b: delish
+      c: yum
+    - a: apple
+      b: ew
+  z: donkey
+bParent:
+  array:
+    - 3
+    - 1
+    - 2
+  c: dog
+```
+

pkg/yqlib/doc/headers/Sort Keys.md

@@ -0,0 +1,9 @@
+The Sort Keys operator sorts maps by their keys (based on their string value). This operator does not do anything to arrays or scalars (so you can easily recursively apply it to all maps).
+
+Sort is particularly useful for diffing two different yaml documents:
+
+```bash
+yq eval -i 'sortKeys(..)' file1.yml
+yq eval -i 'sortKeys(..)' file2.yml
+diff file1.yml file2.yml
+```