Add support for meta configuration

[tomas-langer]

Most applications want to combine config sources. The list of sources may not be known at design time, so it would be great to have support for meta configuration.
This could be a file loaded from (by priority):

• location defined by a system property (e.g. javax.config.metaconfig=file:///tmp/javaconfig-meta.txt
• a "known" location on file system (./javaconfig-meta.txt)
• a "known" location on classpath (META-INF/javaconfig-meta.txt

The content would list config sources and their configuration, e.g.:

environment-variables
system-properties
file: optional=true,path=conf/dev.yaml,watch=true
file: optional=true,path=conf/prod.yaml
classpath: resource=default.yaml
classpath: resource=META-INF/javaconfig.properties

With the following "mandatory" config sources supported:
environment-variables
system-properties
classpath
file (optional and path should be supported)

Other configuration can be vendor specific
We should find a better format than clear text (maybe json?), as it is not flexible enough to describe the possible options nicely
This is based on a feedback from microprofile config presentation and on our MP implementation