Parse configuration from a json file

Why a json file? The current love configuration is impossible to read
from anything other than Lua and can't be imported by other Lua modules.
This changes that
Reorder config.json
Use app.config instead of customized templates.

We were using templates to generate new main.lua and conf.lua files.
Instead, just read from app.config for these values. Supports custom
values (see mixpanel and iteration)
Only overwrite the necessary configuration keys
Merge pull request #1381 from hawkthorne/config_parser

Config parser