Extend existing gitinfo config

Instead of replacing, extend the existing gitinfo config. This way, users can add static information to their Grunt config that is git related but not available on a certain system (i.e. Jenkins).

__Consider the following scenario:__
On a Jenkins system I have a grunt task that, based on the git-flow prefix of the current branch triggers different actions. Since I have no access to the build system I can't configure git-flow on this system.
To help other users understand the available options and to have the git-flow configuration available on any system the config looks like this:

```JavaScript
config.gitinfo = {
        local : {
            branch : {
                current : {
                    SHA              : "Current HEAD SHA",
                    shortSHA         : "Current HEAD short SHA",
                    name             : "Current branch name",
                    currentUser      : "Current git user",
                    lastCommitTime   : "Last commit time",
                    lastCommitAuthor : "Last commit author"
                }
            }
        },
        remote : {
            origin : {
                url : "Branch Url"
            }
        },
        gitflow: {
            branch: {
                master: "master",
                develop: "develop"
            },
            prefix: {
                feature: "feature/",
                release: "release/",
                hotfix: "hotfix/",
                versiontag: "Release-",
                support: "support/"
            }
        }
    };
```

The proposed change would replace the descriptive content for `local` and `remote` but leave the `gitflow` section intact.