Add option to override duplicate settings

[thomastrapp]

This pull request adds a new option as suggested in #83: CONFIG_OPTION_ALLOW_OVERRIDES.

libconfig rejects settings with duplicate names by default. With this option enabled, libconfig will instead remove the old setting and add the new one.

host = "example.com";
# remove setting "host" and add new setting "host":
host = "github.com";

My primary use case is using @include to setup defaults and then overriding specific settings.
For example:

unit = {
  # defaults.conf sets "delay" and other settings
  @include "defaults.conf"
  delay = 2000;
};

If there is anything that I can do to get this merged, please let me know.

[hyperrealm]

This looks great, I would just ask that you please update libconfig.texi to document this new flag (and also add this flag to the C++ API).

[thomastrapp]

I have adjusted the documentation for the C and C++ API. I made the assumption that the option will be available in libconfig v1.7.3: (@b{Since @i{v1.7.3}}).

If there is anything else to fix, please let me know. Thank you for merging!