Lamda Support

[arshanaslam1]

### Issue: Parameter Evaluation in Python Function Rendering with Pystache

#### Description
When rendering a Python function with Pystache, parameters are passed as strings without evaluating them within the function context. This limits the ability to dynamically render content based on function evaluations.

#### Example Code Comparison

The example below demonstrates the intended behavior using Chevron, where the `render` method correctly evaluates the function’s return values.

**Pystache's Approach**

The following `render` method in Pystache illustrates how lambdas are handled but lacks parameter evaluation:

```python
def render(self, engine, context):
    values = engine.fetch_section_data(context, self.key)

    parts = []
    for val in values:
        if callable(val):
            # Special case for lambda functions in sections:
            # The lambda should be treated as an arity-1 function and be passed a string with unprocessed content.
            val = val(self.template[self.index_begin : self.index_end])
            val = engine._render_value(val, context, delimiters=self.delimiters)
            parts.append(val)
            continue

        context.push(val)
        parts.append(self.parsed.render(engine, context))
        context.pop()

    return ''.join(parts)

Chevron’s Approach

In contrast, Chevron passes a render function that directly evaluates values within the template. Here’s an example:

import chevron

def first(text, render):
    # Return only the first occurrence of items
    result = render(text)
    return [x.strip() for x in result.split(" || ") if x.strip()][0]

def inject_x(text, render):
    # Inject data into scope
    return render(text, {'x': 'data'})

args = {
    'template': 'Hello, {{# first}} {{x}} || {{y}} || {{z}} {{/ first}}!  {{# inject_x}} {{x}} {{/ inject_x}}',
    'data': {
        'y': 'foo',
        'z': 'bar',
        'first': first,
        'inject_x': inject_x
    }
}

chevron.render(**args)

[bakert]

Is this a bug (it's against the Mustache spec) or a preference in how it should work? Not ruling out changing it either way I'm just curious.