[SPARK-13049] Add First/last with ignore nulls to functions.scala

This PR adds the ability to specify the ```ignoreNulls``` option to the functions dsl, e.g:
```df.select($"id", last($"value", ignoreNulls = true).over(Window.partitionBy($"id").orderBy($"other"))```

This PR is some where between a bug fix (see the JIRA) and a new feature. I am not sure if we should backport to 1.6.

cc yhuai

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #10957 from hvanhovell/SPARK-13049.