Optionally write comments to config files

[srivathsanmurali]

This PR adds the functionality to optionally add comments while writing configs. This helps give the user an idea about what the setting is.

You can add comments to your settings by using the following API as demonstrated in the example3

For C: We have a new function to add settings

  group = config_setting_add_with_comment(
      root, "address", CONFIG_TYPE_GROUP, "Address for parcel delivery");

For C++, we make use of polymorphism in C++

  Setting &address = root.add("address", Setting::TypeGroup, "Address for parcel delivery");

example3 produces the following output

// Address for parcel delivery
address :
{
  street = "1 Woz Way";
  city = "San Jose";
  state = "CA";
  zip = 95110;
};
numbers = [ 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 ];

[hyperrealm]

__config_setting_destroy() needs to be updated to free this string, otherwise we have a memory leak

[hyperrealm]

should have an overload that takes a const std::string &comment (a simple inline function in libconfig.h++)

doc/libconfig.texi needs to be updated with documentation for the new APIs.

[hyperrealm]

also a couple of unit tests would be nice

[hyperrealm]

We should have a config_setting_set_comment() function as well.

[hyperrealm]

If the comment contains newlines, this will result in a syntax error in the generated file. The comment should be written out such that each line begins with //.