min/max operators (#1992)

* min/max operators

* min, max operator headers

pkg/yqlib/doc/operators/min.md

@@ -0,0 +1,48 @@
+
+## Minimum int
+Given a sample.yml file of:
+```yaml
+- 99
+- 16
+- 12
+- 6
+- 66
+```
+then
+```bash
+yq 'min' sample.yml
+```
+will output
+```yaml
+6
+```
+
+## Minimum string
+Given a sample.yml file of:
+```yaml
+- foo
+- bar
+- baz
+```
+then
+```bash
+yq 'min' sample.yml
+```
+will output
+```yaml
+bar
+```
+
+## Minimum of empty
+Given a sample.yml file of:
+```yaml
+[]
+```
+then
+```bash
+yq 'min' sample.yml
+```
+will output
+```yaml
+```
+