Version 0.1.139

* Implemented dev configuration, close #140
Environment variable
IMPRESS_MODE determine which config file to load, IMPRESS_MODE may be
empty or have any value, "dev" is just for example.
When IMPRESS_MODE
empty or not set, file.js will be loaded and file.dev.js will not be
loaded.
When IMPRESS_MODE=dev, file.dev.js will be loaded and file.js
will not be loaded except the case when file.dev.js is not exist.