substring support

This is a feature request:

can we use jmespath to perform a substring on a string field?

I am thinking something like this:

Input:
```
{
"foo": "someString"
}
```

`substring(0,4,foo) -> 'some'`

The request describes adding substring support to JMESPath and gives the input object plus the expected `substring(0,4,foo)` result. Start by locating the function-definition and expression-test entry points in this Python implementation; done means the proposed expression works for the supplied example and its behavior is covered by tests.

Escrito por el modelo de indexación a partir del texto del issue.