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.
Whatever is easier for you is fine - you could do the changes as another commit on this pull request, or submit a completely separate pull request.