Scenario: Fork the repository with new remote name specified

Allows the user to choose the remote name for their fork instead of
always using their GitHub username. This is useful for workflows where
folks name the remote for their private fork origin `origin` and
the upstream repo remote is named `upstream`. We happen to use this
workflow at work, because it makes the remote names more predictable and
easy to manage (you always push your new commits to `origin` rather than
a remote name that is dependent on your username).

```
$ git remote -v
upstream	git@github.com:github/hub (fetch)
upstream	git@github.com:github/hub (push)

$ ./bin/hub fork --remote-name=origin
Updating origin
From ssh://github.com/github/hub
 * [new branch]      1.11-stable     -> origin/1.11-stable
 * [new branch]      1.12-stable     -> origin/1.12-stable
 * [new branch]      2.2-stable      -> origin/2.2-stable
 * [new branch]      code-coverage   -> origin/code-coverage
 * [new branch]      gh-pages        -> origin/gh-pages
 * [new branch]      master          -> origin/master
 * [new branch]      travis-packages -> origin/travis-packages
new remote: origin

$ git remote -v
origin	git@github.com:msabramo/hub.git (fetch)
origin	git@github.com:msabramo/hub.git (push)
upstream	git@github.com:github/hub (fetch)
upstream	git@github.com:github/hub (push)
```

I haven't written tests yet, but I can if this is a feature that the
maintainers are amenable to merging...

These are very old projects from another time. Today, it doesn't make
sense to try to list similar projects, as they are easily discoverable
through other means. In the recent years, hub hasn't taken direct
inspiration from projects other than git.

Closes #1345

Add option to show pull requests again

Add a `--org` flag to `hub fork`

This change adds a long-form flag `--show-pulls` to `hub issue` that
directs hub to include pull requests in the issue listing.  The
default value is false; the default behavior does not change.

Temporary fix until a better abstraction for pull requests can be
designed.

Related: #1303
Related: vermiculus/magithub#44

Fixes #656

Set merge configuration on checkout.

Fixes #1376, fixes #821  

When checking out a pull request that doesn't match an existing remote,
this sets the branch's remote and merge configuration fields. If the
pull request is modifiable by the maintainer, it uses the SSH URL to
that you can push to the pull request.

cannot find hub completion when using git alias

Add clipboard copy (-c) to browse command

Fix <USER> in usage string

The -c parameter will put the url into the clipboard instead of
opening in the browser.

Since we're no longer connected to a tty, we can't start up tmux.

Add --set-upstream to git push commands.

This ensures that a branch pushed using "hub pull-request" has its
upstream branch set to the target of the push.

We might bring back the self-updater at some point, but right now it's
dead code and it's not clear to me how to implement it in a fashion
where it also updates dependent files such as shell completion scripts
and man pages.

Use --cmd instead of -c to set default filetype for vim

Always start template lookup from the toplevel directory of the repo.

Fixes #1242

Closes #1332, fixes #1305