Allow specify command line flags from environ
Allow specify command line flags from environ

With this option, command line flags can be specified via environment variables like `FAVA_HOST`.

Close #945

aumayr/beancount-web,yagebu/fava,beancount/fava,aumayr/beancount-web,beancount/fava,aumayr/beancount-web,yagebu/fava,yagebu/fava,beancount/fava,beancount/fava,beancount/fava,aumayr/beancount-web,yagebu/fava,yagebu/fava