Feature: Adding Custom config file provision.

This patch takes care of issue #41
Adds a way to browse custom config file.
I've tried to make minimal changes to existing ui.
This patch only does this for local encfs volumes as of now.Tried for dropbox and drive too but for some unknown reason couldn't connect to any cloud accounts here. Would definitely add once I figure out the issue.
Please document this - it's helpful for readers to provide some more context into the class fields.