Enable links on commits to github and bitbucket revision and compare views

This change enables you to link to a repository in either GitHub or BitBucket, to view the changes there. 

When a single commit is pushed, the link will show you that one commit. 
When multiple commits are made, the link will send you to the compare view.

You can enable this by optionally setting:
`git config hooks.slack.github-repo 'myname/my-repo` OR
`git config hooks.slack.bitbucket-repo 'myname/my-repo`

for example, set it to: 
`git config hooks.slack.github-repo 'chriseldredge/git-slack-hook`
to generate links to 
`https://github.com/chriseldredge/git-slack-hook/commit/[hash]`

The result will be:

![image](https://cloud.githubusercontent.com/assets/1104561/3904027/9ec2476c-22d3-11e4-8f8c-d7f1dabb9a8c.png)