Enhancement: Provide a timeline view of PR events

[rkclark]

Description of enhancement

The current dashboard view is very useful, but sometimes it would be beneficial to get an overview of what has happened over a period of time - i.e. the past hour or day.

A timeline view could show all relevant events regarding the watched repos.

It would be useful to see when pull requests are closed. To do so it will be necessary to extend the data that Pullp is requesting from Github, or to keep track of when a PR was previously listed as open and then was no longer present in the data received from the API.

Rationale

Ability to focus on a period in time, could be useful to catch up on the actions in your team.
Gives the user different options for how they use Pullp.