Allow archiving tasks

[cheshire137]

• Fixes #5 by hiding a task from the list if it is archived.
• Persists archived and snoozed tasks so they remain archived/snoozed when you exit and restart the app.
• Adds a refresh button so you can fetch the latest issues and pull requests matching your filter from GitHub.
• Uses a pull request icon for pull requests, instead of an issue icon.
• Displays the repository each issue/pull request is coming from.

Here's a sample of what the persistent JSON storage file looks like:

% cat /Users/cheshire137/Library/Application\ Support/Electron/config.json
{
    "Mentions me": "is:open mentions:cheshire137",
    "rules": [
        "Mentions me"
    ],
    "issue-92485268": "2016-08-27T18:23:57.079Z",
    "archived": [
        "issue-92485268"
    ]
}

/cc @probablycorey

[cheshire137]

Landing this using @probablycorey's rule of "you can land it anyway if I haven't gotten around to looking at it."

In we gooooooo!