Add a new method with_section_content for render_file

This will cover rendering ini file and check if a line is existing under a specific section. Example:

Rendered file:   
``` 
[section1]
option1 = value1
[section2]
option1 = value2
```
chefspec code:
```
render_file(file).with_section_content('section1', 'option1 = value1')
```