Add a way to have pre-configured conf files

Add the ability to put a project-based file with options in yaml, similar to ~/.gem/credentials but for gem-release. Then use the options found in that file, when using gem-release.

Here's an example, using this branch on trying to release gem-release's gemspec, with a `:host` set in the project dir:

<pre>
gem-release ey$ cat .gem-release 
---
:host: http://password:clever@private-gem.server.io

gem-release ey$ gem release *gemspec
...
  File: gem-release-0.7.0.gem
Pushing gem to http://password:clever@private-gem.server.io...
ERROR:  While executing gem ... (SocketError)
    getaddrinfo: nodename nor servname provided, or not known
</pre>

As you can see, it's trying to use the `:host` setting in the project-settings file. 

Added `configuration_test.rb` that tests this new diff, which you should see below. All other tests still pass.