Investigate some potential edge cases around the 'gcfg:"extra_values"' functionality.

[tiagovtristao]

Recent functionality was added around the gcfg:"extra_values" tag which allows extra fields set on the configuration file to be read and set, even if they aren't statically defined.

There a couple of things worth nothing with the gcfg:"extra_values" tag that are worth investigating:

• The original library already supported the gcfg tag which allowed for the variable name in the configuration file to differ from the actual Go struct field. ie. "BuildFilename string gcfg:"anything"" would map the anything variable in the configuration file to the BuildFilename struct field. This means that gcfg:"extra_values" is also a valid variable rename case.
• If a field is set as "MyField map[string]string gcfg:"extra_values"", what happens if there's a corresponding myfield variable set in the configuration file?

These are edge cases that I bumped into when working on separate tickets.