Add default value to Command#initialize param

... so that such instances can be initialized from everywhere without
passing empty arrays.
Move pull_uri up one step and extract issue_uri

Both sit now in the Command class, the parent of all - all subclasses
have access to it now.
Make pr writable, show_pull_request readable in PR

This way other objects have access to them, the regular Show class to
display issues might be interested in them...
Flexibly show issues or pull requests in Show

Show now displays issues or pull requests (it did that before, but as
pull requests have a little more information, a difference needs to be
made).
The requested issue number gets used in a parallel dual call to the
API: Both the issue and the pull API's are hit. If a pull request was
successfully retrieved, it is shown instead of the plain issue view.

The user won't notice the difference when he makes the call, but will
rejoice upon the better PR output in his terminal.
Better help string for ghi show <issueno> -w
Refactor Editor#extract_keyword

We can immediately return after a failing sub!, as this returns nil.