add custom plugins

[bukinoshita]

I want to keep Taskr simple but so many people are asking for new features. To be able to do that I was thinking about custom plugins where each user can install the pluging they prefer.

Example:

• User wants Slack reminders -> Install taskr-slack plugin
• User wants Github issues integration -> Install taskr-github-issues plugin
• User wants Continuous integration -> Install taskr-travisci plugin

That way I can keep Taskr simple and users can customize the way they want