Dynamic host/port config

[Aris Pikeas]

Currently, start_link/0 calls Fluxster.config_for/1. This seems to prevent runtime configuration of parameters.

Here are a few ways to improve this:

• Add @defoverridable start_link: 0
• Make Fluxster.config_for the default value for a new config_fun variable.
• Have config_for read from System.get_env before Application.get_all_env.
• Move config_for into __using__ and make it overridable, and call it on the module rather than on Fluxter.

[Cristian Rosa]

I've submitted a pull request to implement such functionality.
#19
Enjoy!