ENH: enable BlockDX to startup using an alternative config source than the standard AWS one

[walkjivefly]

It is currently possible to test BlockDX with a local set of custom configuration files (eg: when testing new coin integration or updates) but it is quite difficult. It would be easier if BlockDX would accept an optional startup parameter giving the source of configuration data to read instead of the "production" AWS files. This would open up the testing field to many more people since they'd only need to run the regular official release BlockDX and not run a development build.

In discussion with @rBurgett we have decided to provide both an optional startup parameter and an optional environment variable. If neither are provided then BlockDX should start up with the regular production AWS sourced config files. If both are provided then the startup parameter should override the environment variable. When running with a non-default config source this state and the actual configuration files source should be clearly shown in the configuration dialogues.

While this facility is designed with simplifying testing in mind, it could also be useful if anything nasty happens to AWS, like a denial of service attack.

Tasks
[ ] provide an optional startup parameter to override the configuration files source
[ ] provide an environment variable to override the configuration files source
[ ] clearly display in the configuration dialog when a non-standard source is in use, and what the value is
[ ] QA

[walkjivefly]

This should be a higher priority task than it currently is.
The change would also allow us to workaround situations where the config files on AWS are inacessible/incorrect.