Merge config from custom locations with default configuration

Previously, when one or more custom locations were specified on
@ConfigurationProperties, the configuration loaded from those locations
was used in isolation from the default configuration provided by the
environment. Users have been surprised by this behaviour. For example,
it means that a placeholder used in the custom configuration will not
be resolved against the system properties.

This commit adds a new attribute, merge, to @ConfigurationProperties,
that defaults to true. When merge is true the default property sources
are appended to those that are loaded from the custom locations. When
set to false the custom configuration is used in isolation.

Closes #1301