KeyNotFoundException is thrown when .config file contains a key that is missing from the interface representing the configuration.

[urigrumble]

Steps to reproduce:

1. Follow the instructions in steps 1-3 of the instruction at https://github.com/troygoode/ConfigReader/blob/master/README.markdown

2. Go to the app.config/web.config file and add an entry that is not represented in the interface representing the configuration. For example:

   < add key="MailNotificationSettings.MissingFromInterface" value="Fail" / >

Note: The prefix matching the interface name is required for this issue to reproduce.

1. Run the host program

Actual behavior: KeyNotFoundException is thrown from this line: https://github.com/troygoode/ConfigReader/blob/master/src/ConfigReader/ConfigConverter.cs#L34

Expected behavior: Program runs successfully, ignoring the new entry in the .config file.