if you enter an invalid json object such as:
```
{
server: "http://foo.com"
host: "0.0.0.0"
}
```
while configuring, you get errors but are still able to run forever and the message previously was not as indicative of other potential configuration problems.
Closes#1110.
- Promotes config-loader from a validator, to the central place where configuration state is held
- Allow config-loader two means to be told of config file to be used:
- A preferred first argument passed into Ghost
- A secondary GHOST_CONFIG environmental variable
- Failing to see either of the above passed in, config-loader will continue to use "config.js"
- Config-loader validates the target configuration (unchanged) & then copies that object into it's own exports
- Components needing to read configuration now require config-loader to retrieve the configuration state
- Config file continues to be loaded via require(): this is assumed to be a static json file
