Allow multiple plenti.json config files

[jimafisk]

Some users might want to have different build options in different environments. For example maybe you want one baseurl in a dev env, then another baseurl in stg, and finally no baseurl in prod. In this case you should be able to define 3 config files (each with a different baseurl):

• plenti.dev.json
• plenti.stg.json
• plenti.prod.json

Then during CI point to the correct file specifically:

• plenti build --config=plenti.dev.json
• plenti build --config=plenti.stg.json
• plenti build --config=plenti.prod.json

[jimafisk]

The --config flag is available in v0.5.8