Add Support for Configs

[ArunVenkata]

Add support for generating the url based on configs (Ex. staging, production, demo)

Should reference specifications from the .ghprrc file

An example file would be as follows:

[defaults]
... # default parameters

[config:production]
dest="master"
title="Production Merge: "

[config:staging]
dest="staging"
title="Staging Merge: "

The command could be ghpr --config production (Open to Suggestions!)

Note: It should support adding all other parameters as well, which(if specified) would take preference over the ones present in the .ghprrc file

To save your time and mine, DO NOT start your implementation without discussing your approach in this forum first.

A good PR would include:

• Feature Implementation
• Relevant Tests
• Updates to documentation