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 tidy up this try/catch block - there should be only one catch section since the action to take is common (just catching the 'Exception' type is fine). The action taken should be to set mErrDialogText  (see similar error handling later in the function).