Adds default values for section

[mrzarquon]

Our documentation says to use "" for section if you want top of file
global settings, this defaults to that assumption if the user doesn't
provide a section parameter at all.

[hunner]

Could you update the README as well?

[hunner]

Could you actually say that it 'Defaults to ""' like the other parameters say?

[bmjen]

Ping @mrzarquon. Thanks for the contribution. The fix looks good. However can you make a few updates?

1. The doc update that @hunner requested? It should read something like...

• section: The name of the INI file section in which the setting should be defined. Defaults to "", but you can explicitly add a global section --- settings that appear at the beginning of the file, before any named sections --- by specifying a section name of "".

Same applies to the subsetting definition.

2. Please squash your commits down to 1 commit.
3. Please rebase.

Thanks!