allow specifying config file location

[chris48s]

Closes #512

This can't be a command line argument.
Plugins can register command line flags. That means we need to load plugins before we can parse args, which in turn means we have to parse the config file before we can parse args.
So in order to provide this, we make the setting an env var.

TODO:

• Manual testing for loading config files from arbitrary locations (particularly focussing on relative schema locations)
• Write new unit tests covering this functionality
• Decide if this is a breaking change or not. My gut instinct is it is not, but if it is, batch this up with other breaking changes you want to make into a new major

[chris48s]

I think given v8r<=4.1.0 would only look for a config file in cwd, changing relative paths from being resolved relative to the config file location to being resolved relative to cwd makes no difference to any existing user. This can go into 4.2.0